irqchip/gic-v3-its: Add ability to save/restore ITS state

Some platforms power off GIC logic in suspend, so we need to
save/restore state. The distributor and redistributor registers need
to be handled in firmware code due to access permissions on those
registers, but the ITS registers can be restored in the kernel.

We limit this to systems where the ITS collections are implemented
in HW (as opposed to being backed by memory tables), as they are
the only ones that cannot be dealt with by the firmware.

Signed-off-by: Derek Basehore <dbasehore@chromium.org>
[maz: fixed changelog, dropped DT property, limited to HCC being >0]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>