xen/arm: vgic: Re-order the register emulations to match the memory map

xen/arm: vgic: Re-order the register emulations to match the memory map

It helps to find quickly whether we forgot to emulate a register or not.

At the same time add the missing reserved/implementation defined
registers. All other missing registers will be added in a follow-up if
necessary.

Note that only the distributor register map explicitely say the
size of a register (see 8.8 in ARM IHI 0069A). When the size is not
known, the implementation defined/reserved may not be emulated
correctly.

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