MediaTek MT6397/MT6323 PMIC Keys Device Driver

There are two key functions provided by MT6397/MT6323 PMIC, pwrkey
and homekey. The key functions are defined as the subnode of the function
node provided by MT6397/MT6323 PMIC that is being defined as one kind
of Muti-Function Device (MFD)

For MT6397/MT6323 MFD bindings see:
Documentation/devicetree/bindings/mfd/mt6397.txt

Required properties:
- compatible: "mediatek,mt6397-keys" or "mediatek,mt6323-keys"
- linux,keycodes: See Documentation/devicetree/bindings/input/input.yaml

Optional Properties:
- wakeup-source: See Documentation/devicetree/bindings/power/wakeup-source.txt
- mediatek,long-press-mode: Long press key shutdown setting, 1 for
	pwrkey only, 2 for pwrkey/homekey together, others for disabled.
- power-off-time-sec: See Documentation/devicetree/bindings/input/input.yaml

Example:

	pmic: mt6397 {
		compatible = "mediatek,mt6397";

		...

		mt6397keys: mt6397keys {
			compatible = "mediatek,mt6397-keys";
			mediatek,long-press-mode = <1>;
			power-off-time-sec = <0>;

			power {
				linux,keycodes = <116>;
				wakeup-source;
			};

			home {
				linux,keycodes = <114>;
			};
		};

	};