xen/arm: vgic-v3: Allow AArch32 guest booting with GICv3

xen/arm: vgic-v3: Allow AArch32 guest booting with GICv3

AArch32 guest will use co-processor registers to access the GICv3 (see
8.5 in IHI 0069C). Some of the registers have to be trapped and emulated
(e.g ICC_SGI1R), this is the purpose of this patch.

The rest of the emulation already supports access required for AArch32
so nothing has to be changed there.

Note this is only enabling 32-bit guest using GICv3 on Xen ARM64. Further
work would be required to compile GICv3 and vGICv3 for Xen ARM32.

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