GIC: implement suspend and resume

While in suspend state, the system should not wake up due to triggering
of a non wakeup interrupt. Implement suspend and resume functions to be
called from power management code to switch enabled interrupts between
wakeup set or normal set.

Change-Id: Iaceae286707460eadc5f05c0baef72b43c942777
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
(cherry picked from commit 3f2f06c6205445266aabdb9c843da70a4dd5d22f)