xen/arm: p2m: Introduce p2m_get_page_from_gfn

xen/arm: p2m: Introduce p2m_get_page_from_gfn

In a follow-up patch, we will need to handle get_page_from_gfn
differently for DOMID_XEN. To keep the code simple move the current
content in a new separate helper p2m_get_page_from_gfn.

Note the new helper is not anymore a static inline function as the helper
is quite complex.

Finally, take the opportunity to use typesafe gfn as the change is
minor.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>