hwmon: (ltc2990) add devicetree binding

Add a devicetree binding for the ltc2990 hwmon driver.
Signed-off-by: Tom Levens <tom.levens@cern.ch>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ltc2990.txt

+ltc2990: Linear Technology LTC2990 power monitor
+
+Required properties:
+- compatible: Must be "lltc,ltc2990"
+- reg: I2C slave address
+- lltc,meas-mode:
+	An array of two integers for configuring the chip measurement mode.
+
+	The first integer defines the bits 2..0 in the control register. In all
+	cases the internal temperature and supply voltage are measured. In
+	addition the following input measurements are enabled per mode:
+
+		0: V1, V2, TR2
+		1: V1-V2, TR2
+		2: V1-V2, V3, V4
+		3: TR1, V3, V4
+		4: TR1, V3-V4
+		5: TR1, TR2
+		6: V1-V2, V3-V4
+		7: V1, V2, V3, V4
+
+	The second integer defines the bits 4..3 in the control register. This
+	allows a subset of the measurements to be enabled:
+
+		0: Internal temperature and supply voltage only
+		1: TR1, V1 or V1-V2 only per mode
+		2: TR2, V3 or V3-V4 only per mode
+		3: All measurements per mode
+
+Example:
+
+ltc2990@4c {
+	compatible = "lltc,ltc2990";
+	reg = <0x4c>;
+	lltc,meas-mode = <7 3>;	/* V1, V2, V3, V4 */
+};