regulator: bindings: add MPS mp8859 voltage regulator

The MP8859 from Monolithic Power Systems is a single output dc/dc converter
with voltage control over i2c.
Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Link: https://lore.kernel.org/r/20200106211633.2882-5-m.reichl@fivetechno.deSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mp8859.txt

+Monolithic Power Systems MP8859 voltage regulator
+
+Required properties:
+- compatible: "mps,mp8859";
+- reg: I2C slave address.
+
+Optional subnode for regulator: "mp8859_dcdc", using common regulator
+bindings given in <Documentation/devicetree/bindings/regulator/regulator.txt>.
+
+Example:
+
+	mp8859: regulator@66 {
+		compatible = "mps,mp8859";
+		reg = <0x66>;
+		dc_12v: mp8859_dcdc {
+			regulator-name = "dc_12v";
+			regulator-min-microvolt = <12000000>;
+			regulator-max-microvolt = <12000000>;
+			regulator-boot-on;
+			regulator-always-on;
+		};
+	};