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>