dt-bindings: hwmon: Add adm127x documentation

adm127x are hot-swap controllers that allow a circuit board to be removed from
or inserted into a live backplane. This patch adds the device tree
bindings documentation.
Signed-off-by: Kun Yi <kunyi@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/adm1275.txt

+adm1275 properties
+
+Required properties:
+- compatible: Must be one of the supported compatible strings:
+	- "adi,adm1075" for adm1075
+	- "adi,adm1272" for adm1272
+	- "adi,adm1275" for adm1275
+	- "adi,adm1276" for adm1276
+	- "adi,adm1278" for adm1278
+	- "adi,adm1293" for adm1293
+	- "adi,adm1294" for adm1294
+- reg: I2C address
+
+Optional properties:
+
+- shunt-resistor-micro-ohms
+	Shunt resistor value in micro-Ohm
+
+Example:
+
+adm1272@10 {
+	compatible = "adi,adm1272";
+	reg = <0x10>;
+	shunt-resistor-micro-ohms = <500>;
+};