xen/arm: traps: Introduce a helper to read the hypersivor fault register

xen/arm: traps: Introduce a helper to read the hypersivor fault register

While ARM32 has 2 distinct registers for the hypervisor fault register
(one for prefetch abort, the other for data abort), AArch64 has only
one.

Currently, the logic is open-code but a follow-up patch will require to
read it too. So move the logic in a separate helper and use it instead
of open-coding it.

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