xen/arm32: head: Widen the use of the temporary mapping

xen/arm32: head: Widen the use of the temporary mapping

At the moment, the temporary mapping is only used when the virtual
runtime region of Xen is clashing with the physical region.

In follow-up patches, we will rework how secondary CPU bring-up works
and it will be convenient to use the fixmap area for accessing
the root page-table (it is per-cpu).

Rework the code to use temporary mapping when the Xen physical address
is not overlapping with the temporary mapping.

This also has the advantage to simplify the logic to identity map
Xen.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Tested-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>