dt-bindings: regulator: Add stm32mp1 pwr regulators

Document devicetree bindings for stm32mp1 pwr regulators.
Signed-off-by: Pascal Paillet <p.paillet@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/st,stm32mp1-pwr-reg.txt

+STM32MP1 PWR Regulators
+-----------------------
+
+Available Regulators in STM32MP1 PWR block are:
+  - reg11 for regulator 1V1
+  - reg18 for regulator 1V8
+  - usb33 for the swtich USB3V3
+
+Required properties:
+- compatible: Must be "st,stm32mp1,pwr-reg"
+- list of child nodes that specify the regulator reg11, reg18 or usb33
+  initialization data for defined regulators. The definition for each of
+  these nodes is defined using the standard binding for regulators found at
+  Documentation/devicetree/bindings/regulator/regulator.txt.
+- vdd-supply: phandle to the parent supply/regulator node for vdd input
+- vdd_3v3_usbfs-supply: phandle to the parent supply/regulator node for usb33
+
+Example:
+
+pwr_regulators: pwr@50001000 {
+	compatible = "st,stm32mp1,pwr-reg";
+	reg = <0x50001000 0x10>;
+	vdd-supply = <&vdd>;
+	vdd_3v3_usbfs-supply = <&vdd_usb>;
+
+	reg11: reg11 {
+		regulator-name = "reg11";
+		regulator-min-microvolt = <1100000>;
+		regulator-max-microvolt = <1100000>;
+	};
+
+	reg18: reg18 {
+		regulator-name = "reg18";
+		regulator-min-microvolt = <1800000>;
+		regulator-max-microvolt = <1800000>;
+	};
+
+	usb33: usb33 {
+		regulator-name = "usb33";
+		regulator-min-microvolt = <3300000>;
+		regulator-max-microvolt = <3300000>;
+	};
+};