irqchip/gic-pm: Fix suspend handling

If interrupts are enabled for a non-root GIC device that uses the
gic-pm driver, when system suspend occurs, the current interrupt
state is not saved and restored correctly and so interrupts do not
work again on resuming the system. Add a late suspend handler to
save and restore the state for these devices.

Suggested-by: Jonathan Hunter <jonathanh@nvidia.com>
Signed-off-by: Sameer Pujar <spujar@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>