drivers: Remove useless trailing comments from mmiowb() invocations

In preparation for using coccinelle to remove all mmiowb() instances
from drivers, remove all trailing comments since they won't be picked up
by spatch later on and will end up being preserved in the code.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>