xen/arm: Add support for read-only foreign mappings

xen/arm: Add support for read-only foreign mappings

Currently, foreign mappings can only be read-write. A follow-up patch will
extend foreign mapping for Xen backend memory (via XEN_DOMID), some of
that memory should only be read accessible for the mapping domain.

Introduce a new p2m_type to cater read-only foreign mappings. For now,
the decision between the two foreign mapping type is based on the type
of the guest page mapped.

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