xen/arm: traps: Move MMIO emulation code in a separate helper

xen/arm: traps: Move MMIO emulation code in a separate helper

Currently, a stage-2 fault translation will likely access an emulated
region. All the checks are pre-sanitity check for MMIO emulation.

A follow-up patch will handle a new case that could lead to a stage-2
translation. To improve the clarity of the code and the changes, the
current implementation is move in a separate helper.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>