ARM: new VGIC: Add IRQ sync/flush framework

ARM: new VGIC: Add IRQ sync/flush framework

Implement the framework for syncing IRQs between our emulation and the
list registers, which represent the guest's view of IRQs.
This is done in vgic_sync_from_lrs() and vgic_sync_to_lrs(), which
get called on guest entry and exit, respectively.
The code talking to the actual GICv2/v3 hardware is added in the
following patches.

This is based on Linux commit 0919e84c0fc1, written by Marc Zyngier.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>