Device Tree Bindings for LED support on MT6323 PMIC

MT6323 LED controller is subfunction provided by MT6323 PMIC, so the LED
controllers are defined as the subnode of the function node provided by MT6323
PMIC controller that is being defined as one kind of Muti-Function Device (MFD)
using shared bus called PMIC wrapper for each subfunction to access remote
MT6323 PMIC hardware.

For MT6323 MFD bindings see:
Documentation/devicetree/bindings/mfd/mt6397.txt
For MediaTek PMIC wrapper bindings see:
Documentation/devicetree/bindings/soc/mediatek/mediatek,pwrap.yaml

Required properties:
- compatible : Must be one of
  - "mediatek,mt6323-led"
  - "mediatek,mt6331-led"
  - "mediatek,mt6332-led"
- address-cells : Must be 1
- size-cells : Must be 0

Each led is represented as a child node of the mediatek,mt6323-led that
describes the initial behavior for each LED physically and currently only four
LED child nodes can be supported.

Required properties for the LED child node:
- reg : LED channel number (0..3)

Optional properties for the LED child node:
- label : See Documentation/devicetree/bindings/leds/common.txt
- linux,default-trigger : See Documentation/devicetree/bindings/leds/common.txt
- default-state: See Documentation/devicetree/bindings/leds/common.txt

Example:

	mt6323: pmic {
		compatible = "mediatek,mt6323";

		...

		mt6323led: leds {
			compatible = "mediatek,mt6323-led";
			#address-cells = <1>;
			#size-cells = <0>;

			led@0 {
				reg = <0>;
				label = "LED0";
				linux,default-trigger = "timer";
				default-state = "on";
			};
			led@1 {
				reg = <1>;
				label = "LED1";
				default-state = "off";
			};
			led@2 {
				reg = <2>;
				label = "LED2";
				default-state = "on";
			};
		};
	};