input: Add MediaTek PMIC keys support

This patch add support to handle MediaTek PMIC MT6397/MT6323 key
interrupts including pwrkey and homekey, also add setting for
long press key shutdown behavior.

Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>