dt-bindings: input: add common keyboard document bindings

This patch adds the device tree binding documentation for common
keyboard.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/devicetree/bindings/input/keys.txt

+General Keys Properties:
+
+Optional properties for Keys:
+- power-off-time-sec: Duration in seconds which the key should be kept
+	pressed for device to power off automatically. Device with key pressed
+	shutdown feature can specify this property.
+- linux,keycodes: Specifies the numeric keycode values to be used for
+	reporting key presses.