hwmon: (ltc2978) device tree bindings documentation

Add device tree bindings documentation for ltc2978.
Signed-off-by: Alan Tull <atull@opensource.altera.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>
[Guenter Roeck: Minor correction of 'compatible' example]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/devicetree/bindings/hwmon/ltc2978.txt

+ltc2978
+
+Required properties:
+- compatible: should contain one of:
+  * "lltc,ltc2974"
+  * "lltc,ltc2977"
+  * "lltc,ltc2978"
+  * "lltc,ltc3880"
+  * "lltc,ltc3883"
+  * "lltc,ltm4676"
+- reg: I2C slave address
+
+Optional properties:
+- regulators: A node that houses a sub-node for each regulator controlled by
+  the device. Each sub-node is identified using the node's name, with valid
+  values listed below. The content of each sub-node is defined by the
+  standard binding for regulators; see regulator.txt.
+
+Valid names of regulators depend on number of supplies supported per device:
+  * ltc2974 : vout0 - vout3
+  * ltc2977 : vout0 - vout7
+  * ltc2978 : vout0 - vout7
+  * ltc3880 : vout0 - vout1
+  * ltc3883 : vout0
+  * ltm4676 : vout0 - vout1
+
+Example:
+ltc2978@5e {
+	compatible = "lltc,ltc2978";
+	reg = <0x5e>;
+	regulators {
+		vout0 {
+			regulator-name = "FPGA-2.5V";
+		};
+		vout2 {
+			regulator-name = "FPGA-1.5V";
+		};
+	};
+};