Documentation: mfd: Add DT bindings for the LP873X

The lp873x series of PMICs have a bunch of regulators and a couple
of GPO(General Purpose Outputs).
Add information for the MFD and regulator drivers.
Signed-off-by: NKeerthy <j-keerthy@ti.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/lp873x.txt

+TI LP873X PMIC MFD driver
+
+Required properties:
+  - compatible:	"ti,lp8732", "ti,lp8733"
+  - reg:		I2C slave address.
+  - gpio-controller:	Marks the device node as a GPIO Controller.
+  - #gpio-cells:	Should be two.  The first cell is the pin number and
+			the second cell is used to specify flags.
+			See ../gpio/gpio.txt for more information.
+  - regulators:	List of child nodes that specify the regulator
+			initialization data.
+Example:
+
+pmic: lp8733@60 {
+	compatible = "ti,lp8733";
+	reg = <0x60>;
+	gpio-controller;
+	#gpio-cells = <2>;
+
+	regulators {
+		lp8733_buck0: buck0 {
+			regulator-name = "lp8733-buck0";
+			regulator-min-microvolt = <800000>;
+			regulator-max-microvolt = <1400000>;
+			regulator-min-microamp = <1500000>;
+			regulator-max-microamp = <4000000>;
+			regulator-ramp-delay = <10000>;
+			regulator-always-on;
+			regulator-boot-on;
+		};
+
+		lp8733_buck1: buck1 {
+			regulator-name = "lp8733-buck1";
+			regulator-min-microvolt = <800000>;
+			regulator-max-microvolt = <1400000>;
+			regulator-min-microamp = <1500000>;
+			regulator-max-microamp = <4000000>;
+			regulator-ramp-delay = <10000>;
+			regulator-boot-on;
+			regulator-always-on;
+		};
+
+		lp8733_ldo0: ldo0 {
+			regulator-name = "lp8733-ldo0";
+			regulator-min-microvolt = <800000>;
+			regulator-max-microvolt = <3000000>;
+			regulator-boot-on;
+			regulator-always-on;
+		};
+
+		lp8733_ldo1: ldo1 {
+			regulator-name = "lp8733-ldo1";
+			regulator-min-microvolt = <800000>;
+			regulator-max-microvolt = <3000000>;
+			regulator-always-on;
+			regulator-boot-on;
+		};
+	};
+};