input: Add support for Qualcomm PMIC8XXX power key

Add support for PMIC8XXX power key driven over dedicated
KYPD_PWR_N pin.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>