move UL() macro from armv8/mmu.h into common.h

The UL() macro is pretty useful in sharing constants between assembly
and C files while still being able to specify a type for C.
Move the macro from an armv8 specific header into a common header file
to be able to use it by arm code (for instance) as well.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Jagan Teki <jagan@openedev.com>