ARM: implement create_mapping_late() for EFI use

This implements create_mapping_late(), which we will use to populate
the UEFI Runtime Services page tables.

Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>