GICv3: ITS architectural save and restore helpers

GICv3: ITS architectural save and restore helpers

This patch adds functions to save and restore GICv3 ITS registers during
system suspend. Please note that the power management of GIC ITS is
implementation defined. These functions only implements the
architectural part of the ITS power management and they do not restore
memory structures or register content required to support ITS. Even if
the ITS implementation stores structures in memory, an implementation
defined power down sequence is likely to be required to flush some
internal ITS caches to memory. If such implementation defined sequence
is not followed, the platform must ensure that the ITS is not power
gated during system suspend.

Change-Id: I5f31e5541975aa7dcaab69b0b7f67583c0e27678
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>