regulator: 88pg86x: add DT bindings document

The only device-specific node names are "buck1" and "buck2" for the two
regulators present on the device. Sleep mode GPIO and per-regulator GPIO
enable pins are not exposed (the driver does not support them either).
Signed-off-by: NAlexander Monakov <amonakov@ispras.ru>
Signed-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/88pg86x.txt

+Marvell 88PG867/88PG868 voltage regulators
+
+Required properties:
+- compatible: one of "marvell,88pg867", "marvell,88pg868";
+- reg: I2C slave address.
+
+Optional subnodes for regulators: "buck1", "buck2", using common regulator
+bindings given in <Documentation/devicetree/bindings/regulator/regulator.txt>.
+
+Example:
+
+	pg868@19 {
+		compatible = "marvell,88pg868";
+		reg = <0x19>;
+
+		vcpu: buck1 {
+			regulator-boot-on;
+			regulator-always-on;
+			regulator-min-microvolt = <1000000>;
+			regulator-max-microvolt = <1350000>;
+		};
+	};