xen/arm: page: Remove unused attributes DEV_NONSHARED and DEV_CACHED

xen/arm: page: Remove unused attributes DEV_NONSHARED and DEV_CACHED

They were imported from non-LPAE Linux, but Xen is LPAE only. It is time
to do some clean-up in the memory attribute and keep only what make
sense for Xen. Follow-up patch will do more clean-up.

Also, update the comment saying our attribute matches Linux.

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