Input: mcs_touchkey - add shutdown handler

The MCS50XX series has a HW bug that requires explicit chip power down.
If chip is not powered down before shutting the system down the control
pins (powerup, interrupt) are pulled up and residue current continues
flowing into the chips making them continue consuming power.

Signed-off-by: Heungjun Kim <riverful.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>