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

This patch adds the device tree binding documentation for the MediaTek
pmic keys found on PMIC MT6397/MT6323.
Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/input/mtk-pmic-keys.txt

+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>;
+			};
+		};
+
+	};