xen/arm: vgic: Properly emulate the full register

xen/arm: vgic: Properly emulate the full register

The offset in the emulation is based on byte. As most of the registers
are 64/32 bits, they will span over multiple bytes.

However, the current emulation only cares about the first offset. This
will result in not properly emulating any access on the register with
any other offset.

Introduce new macros to help implementing access on multiple byte and
use them over the vGIC emulation.

Note that I didn't convert the reserved/implementation defined
registers. It will be done in a follow-up.

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