Documentation/ABI: Add documentation mlxreg-io sysfs interfaces

Add documentation for mlxreg-io platform driver sysfs interfaces to allow
user space access for system resets control, reset causes monitoring,
programmable devices version reading and device selection control.
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>

Documentation/ABI/stable/sysfs-driver-mlxreg-io

+What:		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/
+							asic_health
+
+Date:		June 2018
+KernelVersion:	4.19
+Contact:	Vadim Pasternak <vadimpmellanox.com>
+Description:	This file shows ASIC health status. The possible values are:
+		0 - health failed, 2 - health OK, 3 - ASIC in booting state.
+
+		The files are read only.
+
+What:		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/
+							cpld1_version
+							cpld2_version
+
+Date:		June 2018
+KernelVersion:	4.19
+Contact:	Vadim Pasternak <vadimpmellanox.com>
+Description:	These files show with which CPLD versions have been burned
+		on carrier and switch boards.
+
+		The files are read only.
+
+What:		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/select_iio
+Date:		June 2018
+KernelVersion:	4.19
+Contact:	Vadim Pasternak <vadimpmellanox.com>
+Description:	This file allows iio devices selection.
+
+		Attribute select_iio can be written with 0 or with 1. It
+		selects which one of iio devices can be accessed.
+
+		The file is read/write.
+
+What:		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/psu1_on
+		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/psu2_on
+		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/pwr_cycle
+		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/pwr_down
+Date:		June 2018
+KernelVersion:	4.19
+Contact:	Vadim Pasternak <vadimpmellanox.com>
+Description:	These files allow asserting system power cycling, switching
+		power supply units on and off and system's main power domain
+		shutdown.
+		Expected behavior:
+		When pwr_cycle is written 1: auxiliary power domain will go
+		down and after short period (about 1 second) up.
+		When  psu1_on or psu2_on is written 1, related unit will be
+		disconnected from the power source, when written 0 - connected.
+		If both are written 1 - power supplies main power domain will
+		go down.
+		When pwr_down is written 1, system's main power domain will go
+		down.
+
+		The files are write only.
+
+What:		/sys/devices/platform/mlxplat/mlxreg-io/hwmon/hwmon*/
+							reset_aux_pwr_or_ref
+							reset_asic_thermal
+							reset_hotswap_or_wd
+							reset_fw_reset
+							reset_long_pb
+							reset_main_pwr_fail
+							reset_short_pb
+							reset_sw_reset
+Date:		June 2018
+KernelVersion:	4.19
+Contact:	Vadim Pasternak <vadimpmellanox.com>
+Description:	These files show the system reset cause, as following: power
+		auxiliary outage or power refresh, ASIC thermal shutdown,
+		hotswap or watchdog, firmware reset, long press power button,
+		short press power button, software reset. Value 1 in file means
+		this is reset cause, 0 - otherwise. Only one of the above
+		causes could be 1 at the same time, representing only last
+		reset cause.
+
+		The files are read only.