docs: hwmon: (smpro-hwmon) Add documentation

Add documentation for the Ampere(R)'s Altra(R) SMpro hwmon driver.
Signed-off-by: Thu Nguyen <thu@os.amperecomputing.com>
Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
Link: https://lore.kernel.org/r/20220929094321.770125-3-quan@os.amperecomputing.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -187,6 +187,7 @@ Hardware Monitoring Kernel Drivers
    sis5595
    sl28cpld
    smm665
+   smpro-hwmon
    smsc47b397
    smsc47m192
    smsc47m1

Documentation/hwmon/smpro-hwmon.rst

+.. SPDX-License-Identifier: GPL-2.0-only
+
+Kernel driver Ampere(R)'s Altra(R) SMpro hwmon
+==============================================
+
+Supported chips:
+
+  * Ampere(R) Altra(R)
+
+    Prefix: 'smpro'
+
+    Reference: Altra SoC BMC Interface Specification
+
+Author: Thu Nguyen <thu@os.amperecomputing.com>
+
+Description
+-----------
+This driver supports hardware monitoring for Ampere(R) Altra(R) SoC's based on the
+SMpro co-processor (SMpro).
+The following sensor types are supported by the driver:
+
+  * temperature
+  * voltage
+  * current
+  * power
+
+The SMpro interface provides the registers to query the various sensors and
+their values which are then exported to userspace by this driver.
+
+Usage Notes
+-----------
+
+SMpro hwmon driver creates at least two sysfs files for each sensor.
+
+* File ``<sensor_type><idx>_label`` reports the sensor label.
+* File ``<sensor_type><idx>_input`` returns the sensor value.
+
+The sysfs files are allocated in the SMpro root fs folder.
+There is one root folder for each SMpro instance.
+
+When the SoC is turned off, the driver will fail to read registers
+and return -ENXIO.
+
+Sysfs entries
+-------------
+
+The following sysfs files are supported:
+
+* Ampere(R) Altra(R):
+
+============    =============   ======  ===============================================
+Name            Unit            Perm    Description
+temp1_input     milli Celsius   RO      SoC temperature
+temp2_input     milli Celsius   RO      Max temperature reported among SoC VRDs
+temp2_crit      milli Celsius   RO      SoC VRD HOT Threshold temperature
+temp3_input     milli Celsius   RO      Max temperature reported among DIMM VRDs
+temp4_input     milli Celsius   RO      Max temperature reported among Core VRDs
+temp5_input     milli Celsius   RO      Temperature of DIMM0 on CH0
+temp5_crit      milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp6_input     milli Celsius   RO      Temperature of DIMM0 on CH1
+temp6_crit      milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp7_input     milli Celsius   RO      Temperature of DIMM0 on CH2
+temp7_crit      milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp8_input     milli Celsius   RO      Temperature of DIMM0 on CH3
+temp8_crit      milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp9_input     milli Celsius   RO      Temperature of DIMM0 on CH4
+temp9_crit      milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp10_input    milli Celsius   RO      Temperature of DIMM0 on CH5
+temp10_crit     milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp11_input    milli Celsius   RO      Temperature of DIMM0 on CH6
+temp11_crit     milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp12_input    milli Celsius   RO      Temperature of DIMM0 on CH7
+temp12_crit     milli Celsius   RO      MEM HOT Threshold for all DIMMs
+temp13_input    milli Celsius   RO      Max temperature reported among RCA VRDs
+in0_input       milli Volts     RO      Core voltage
+in1_input       milli Volts     RO      SoC voltage
+in2_input       milli Volts     RO      DIMM VRD1 voltage
+in3_input       milli Volts     RO      DIMM VRD2 voltage
+in4_input       milli Volts     RO      RCA VRD voltage
+cur1_input      milli Amperes   RO      Core VRD current
+cur2_input      milli Amperes   RO      SoC VRD current
+cur3_input      milli Amperes   RO      DIMM VRD1 current
+cur4_input      milli Amperes   RO      DIMM VRD2 current
+cur5_input      milli Amperes   RO      RCA VRD current
+power1_input    micro Watts     RO      Core VRD power
+power2_input    micro Watts     RO      SoC VRD power
+power3_input    micro Watts     RO      DIMM VRD1 power
+power4_input    micro Watts     RO      DIMM VRD2 power
+power5_input    micro Watts     RO      RCA VRD power
+============    =============   ======  ===============================================
+
+Example::
+
+    # cat in0_input
+    830
+    # cat temp1_input
+    37000
+    # cat curr1_input
+    9000
+    # cat power5_input
+    19500000