Input: sh_keysc - implement runtime PM support

Convert the KEYSC driver from MSTP bit control through a magic clock
string to making use of Runtime PM.

Also, update the system suspend/resume callbacks to drop the Runtime
PM reference in the case of wakeup disabled.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>