ARM: new VGIC: Add GICv2 MMIO handling framework

ARM: new VGIC: Add GICv2 MMIO handling framework

Create vgic-mmio-v2.c to describe GICv2 emulation specific handlers
using the initializer macros provided by the VGIC MMIO framework.
Provide a function to register the GICv2 distributor registers to
the Xen MMIO framework.
The actual handler functions are still stubs in this patch.

This is based on Linux commit fb848db39661, written by Andre Przywara.

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