Flush the GIC driver data after init

The GIC driver data is initialized by the primary CPU with caches
enabled. When the secondary CPU boots up, it initializes the
GICC/GICR interface with the caches disabled and there is a chance that
the driver data is not yet written back to the memory. This patch fixes
this problem by flushing the driver data after they have been
initialized.

Change-Id: Ie9477029683846209593ff005d2bac559bb8f5e6
Signed-off-by: Soby Mathew <soby.mathew@arm.com>