regulator: add binding for the SY8106A voltage regulator

SY8106A is an I2C-controlled adjustable voltage regulator made by
Silergy Corp.

Add its device tree binding.
Signed-off-by: Ondrej Jirman <megous@megous.com>
[Icenowy: Change commit message and slight fixes]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/sy8106a-regulator.txt

+SY8106A Voltage regulator
+
+Required properties:
+- compatible: Must be "silergy,sy8106a"
+- reg: I2C slave address - must be <0x65>
+- silergy,fixed-microvolt - the voltage when I2C regulating is disabled (set
+  by external resistor like a fixed voltage)
+
+Any property defined as part of the core regulator binding, defined in
+./regulator.txt, can also be used.
+
+Example:
+
+	sy8106a {
+		compatible = "silergy,sy8106a";
+		reg = <0x65>;
+		regulator-name = "sy8106a-vdd";
+		silergy,fixed-microvolt = <1200000>;
+		regulator-min-microvolt = <1000000>;
+		regulator-max-microvolt = <1400000>;
+		regulator-boot-on;
+		regulator-always-on;
+	};