xen/arm: mm: Allow page-table allocation from the boot allocator

xen/arm: mm: Allow page-table allocation from the boot allocator

At the moment, page-table can only be allocated from domheap. This means
it is not possible to create mapping in the page-tables via
map_pages_to_xen() if page-table needs to be allocated.

In order to avoid open-coding page-tables update in early boot, we need
to be able to allocate page-tables much earlier. Thankfully, we have the
boot allocator for those cases.

create_xen_table() is updated to cater early boot allocation by using
alloc_boot_pages().

Note, this is not sufficient to bootstrap the page-tables (i.e mapping
before any memory is actually mapped). This will be addressed
separately.

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