regulator: bindings: add MPS mp8869 voltage regulator

The MP8869 from Monolithic Power Systems is a single output dc/dc
converter with voltage control over i2c.
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200316222945.74ad34dd@xhackerSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/mp886x.txt

+Monolithic Power Systems MP8869 voltage regulator
+
+Required properties:
+- compatible: "mps,mp8869";
+- reg: I2C slave address.
+- enable-gpios: enable gpios.
+- mps,fb-voltage-divider: An array of two integers containing the resistor
+  values R1 and R2 of the feedback voltage divider in kilo ohms.
+
+Any property defined as part of the core regulator binding, defined in
+./regulator.txt, can also be used.
+
+Example:
+
+	vcpu: regulator@62 {
+		compatible = "mps,mp8869";
+		regulator-name = "vcpu";
+		regulator-min-microvolt = <700000>;
+		regulator-max-microvolt = <850000>;
+		regulator-always-on;
+		regulator-boot-on;
+		enable-gpios = <&porta 1 GPIO_ACTIVE_LOW>;
+		mps,fb-voltage-divider = <80 240>;
+		reg = <0x62>;
+	};