irq-gic-v3: Disable group pending interrupts before hotplug

Collapsing the cpu with ICC_GRPEN1_EL1 puts GIC in a unknown state where
the cpu might not see the interrupts pending at the redistributor on
its wakeup.

Clear this register on the collapse path.

Change-Id: I5d3a71846feb1a79d091f6adbf4ad39fa9328479
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>