mm: use typesafe MFN for alloc_boot_pages return

mm: use typesafe MFN for alloc_boot_pages return

At the moment, most of the callers will have to use mfn_x. However
follow-up patches will remove some of them by propagating the typesafe a
bit further.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>