This adds GENMASK_U128() tests although currently only 64 bit wide masks
are being tested.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>