xen/arm: mm: Re-implement early_fdt_map() using map_pages_to_xen()

xen/arm: mm: Re-implement early_fdt_map() using map_pages_to_xen()

Now that map_pages_to_xen() has been extended to support 2MB mappings,
we can replace the create_mappings() calls by map_pages_to_xen() calls.

The mapping can also be marked read-only as Xen should not modify
the host Device Tree during boot.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Hongda Deng <Hongda.Heng@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>