xen/arm: mm: Rework setup_xenheap_mappings()

xen/arm: mm: Rework setup_xenheap_mappings()

The current implementation of setup_xenheap_mappings() is using 1GB
mappings. This can lead to unexpected result because the mapping
may alias a non-cachable region (such as device or reserved regions).
For more details see B2.8 in ARM DDI 0487H.a.

map_pages_to_xen() was recently reworked to allow superpage mappings,
support contiguous mapping and deal with the use of page-tables before
they are mapped.

Most of the code in setup_xenheap_mappings() is now replaced with a
single call to map_pages_to_xen().

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>