xen/arm: page: Describe the layout of flags used to update page tables

xen/arm: page: Describe the layout of flags used to update page tables

Currently, the flags used to update page tables (i.e PAGE_HYPERVISOR_*)
only contains the memory attribute index. Follow-up patches will add
more information in it. So document the current layout.

At the same time introduce PAGE_AI_MASK to get the memory attribute
index easily.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>