During system suspend, the GICv3 Distributor and Redistributor context
can be lost due to power gating of the system power domain. This means
that the GICv3 context needs to be saved prior to system suspend and
restored on wakeup. Currently the consensus is that the Firmware should
be in charge of this. See tf-issues#464 for more details.

This patch introduces helper APIs in the GICv3 driver to save and
restore the Distributor and Redistributor contexts. The GICv3 ITS
context is not considered in this patch because the specification says
that the details of ITS power management is implementation-defined.
These APIs are expected to be appropriately invoked by the platform
layer during system suspend.

Fixes ARM-software/tf-issues#464

Change-Id: Iebb9c6770ab8c4d522546f161fa402d2fe02ec00
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>