hwmon: (ads7828) Accept optional parameters from device tree

Adding the ability for the ads7828 and ads7830 to use device tree to
get optional parameters instead of using platform devices. This allows
people using custom boards to also use the ads7828 in a non-default manner.
Signed-off-by: Sam Povilus <kernel.development@povil.us>
[groeck: Fixed whitespace errors in ads7828.txt]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ads7828.txt

+ads7828 properties
+
+Required properties:
+- compatible: Should be one of
+	       ti,ads7828
+	       ti,ads7830
+- reg: I2C address
+
+Optional properties:
+
+- ti,differential-input
+  Set to use the device in differential mode.
+- vref-supply
+  The external reference on the device is set to this regulators output. If it
+  does not exists the internal reference will be used and output by the ads78xx
+  on the "external vref" pin.
+
+  Example ADS7828 node:
+
+  ads7828: ads@48 {
+	   comatible = "ti,ads7828";
+	   reg = <0x48>;
+	   vref-supply = <&vref>;
+	   ti,differential-input;
+  };

drivers/hwmon/ads7828.c

@@ -35,6 +35,7 @@
 #include <linux/platform_data/ads7828.h>
 #include <linux/regmap.h>
 #include <linux/slab.h>
+#include <linux/regulator/consumer.h>
 
 /* The ADS7828 registers */
 #define ADS7828_CMD_SD_SE	0x80	/* Single ended inputs */
@@ -119,10 +120,12 @@ static int ads7828_probe(struct i2c_client *client,
 	struct ads7828_data *data;
 	struct device *hwmon_dev;
 	unsigned int vref_mv = ADS7828_INT_VREF_MV;
+	unsigned int vref_uv;
 	bool diff_input = false;
 	bool ext_vref = false;
 	unsigned int regval;
 	enum ads7828_chips chip;
+	struct regulator *reg;
 
 	data = devm_kzalloc(dev, sizeof(struct ads7828_data), GFP_KERNEL);
 	if (!data)
@@ -133,6 +136,18 @@ static int ads7828_probe(struct i2c_client *client,
 		ext_vref = pdata->ext_vref;
 		if (ext_vref && pdata->vref_mv)
 			vref_mv = pdata->vref_mv;
+	} else if (dev->of_node) {
+		diff_input = of_property_read_bool(dev->of_node,
+						   "ti,differential-input");
+		reg = devm_regulator_get_optional(dev, "vref");
+		if (!IS_ERR(reg)) {
+			vref_uv = regulator_get_voltage(reg);
+			vref_mv = DIV_ROUND_CLOSEST(vref_uv, 1000);
+			if (vref_mv < ADS7828_EXT_VREF_MV_MIN ||
+			    vref_mv > ADS7828_EXT_VREF_MV_MAX)
+				return -EINVAL;
+			ext_vref = true;
+		}
 	}
 
 	if (client->dev.of_node)