dt-bindings: input: Add document bindings for mtk-pmic-keys

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/keys.txt

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/keys.txt

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>;

			};

		};

	};