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 <[e-mail]>
[Guenter Roeck: Added missing of.h include]
Signed-off-by: Guenter Roeck <[e-mail]>

Documentation/devicetree/bindings/i2c/ina2xx.txt

···
+ina2xx properties
+
+Required properties:
+- compatible: Must be one of the following:
+	- "ti,ina219" for ina219
+	- "ti,ina220" for ina220
+	- "ti,ina226" for ina226
+	- "ti,ina230" for ina230
+- reg: I2C address
+
+Optional properties:
+
+- shunt-resistor
+	Shunt resistor value in micro-Ohm
+
+Example:
+
+ina220@44 {
+	compatible = "ti,ina220";
+	reg = <0x44>;
+	shunt-resistor = <1000>;
+};

Documentation/hwmon/ina2xx

···
 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.

drivers/hwmon/ina2xx.c

···
 #include <linux/hwmon.h>
 #include <linux/hwmon-sysfs.h>
 #include <linux/jiffies.h>
+#include <linux/of.h>
 
 #include <linux/platform_data/ina2xx.h>
 
···
 	struct ina2xx_data *data;
 	struct ina2xx_platform_data *pdata;
 	int ret;
+	u32 val;
 	long shunt = 10000; /* default shunt value 10mOhms */
 
 	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_WORD_DATA))
···
 		pdata =
 		  (struct ina2xx_platform_data *)client->dev.platform_data;
 		shunt = pdata->shunt_uohms;
+	} else if (!of_property_read_u32(client->dev.of_node,
+				"shunt-resistor", &val)) {
+			shunt = val;
 	}
 
 	if (shunt <= 0)