xen/arm: vgic-v3: Support 32-bit access for 64-bit registers

xen/arm: vgic-v3: Support 32-bit access for 64-bit registers

Based on 8.1.3 (IHI 0069A), unless stated otherwise, the 64-bit registers
supports both 32-bit and 64-bits access.

All the registers we properly emulate (i.e not RAZ/WI) supports 32-bit access.

For RAZ/WI, it's also seems to be the case but I'm not 100% sure. Anyway,
emulating 32-bit access for them doesn't hurt. Note that we would need
some extra care when they will be implemented (for instance GICR_PROPBASER).

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>