The ARM GIC driver treats the entire contents of the GICC_HPPIR as the interrupt
ID instead of just bits[9:0]. This could result in an SGI being treated as a
Group 1 interrupt on a GICv2 system.

This patch introduces a mask to retrieve only the ID from a read of GICC_HPPIR,
GICC_IAR and similar registers. The value read from these registers is masked
with this constant prior to use as an interrupt ID.

Fixes ARM-software/tf-issues#306

Change-Id: Ie3885157de33b71df9781a41f6ef015a30c4608d