hwmon: (ina2xx) Add device tree support to pass the shunt resistor

Adding another way that is device tree to pass the shunt resistor
value to driver except for platform data.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
[Guenter Roeck: Added missing of.h include]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
diff --git a/Documentation/hwmon/ina2xx b/Documentation/hwmon/ina2xx
index 03444f9..4223c2d 100644
--- a/Documentation/hwmon/ina2xx
+++ b/Documentation/hwmon/ina2xx
@@ -44,4 +44,6 @@
 The INA230 is a high or low side current shunt and power monitor with an I2C
 interface. The INA230 monitors both a shunt voltage drop and bus supply voltage.
 
-The shunt value in micro-ohms can be set via platform data.
+The shunt value in micro-ohms can be set via platform data or device tree.
+Please refer to the Documentation/devicetree/bindings/i2c/ina2xx.txt for bindings
+if the device tree is used.