mfd: Add PM support to STMPE devices

Add PM helpers to STMPE and add support to enable wakeup from low power
states

Acked-by: Rabin VINCENT <rabin.vincent@stericsson.com>
Acked-by: Linus WALLEIJ <linus.walleij@stericsson.com>
Signed-off-by: Sundar Iyer <sundar.iyer@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>