ARM: new VGIC: Add MMIO handling framework

ARM: new VGIC: Add MMIO handling framework

Add an MMIO handling framework to the VGIC emulation:
Each register is described by its offset, size (or number of bits per
IRQ, if applicable) and the read/write handler functions. We provide
initialization macros to describe each GIC register later easily.

Separate dispatch functions for read and write accesses are connected
to Xen's MMIO handling framework and binary-search for the responsible
register handler based on the offset address within the region.

The register handler prototype are courtesy of Christoffer Dall.

This is based on Linux commit 4493b1c4866a, written by Marc Zyngier.

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