Documentation: Kill all references to mmiowb()

The guarantees provided by mmiowb() are now provided implicitly by
spin_unlock(), so remove all references to this most confusing of
barriers from our Documentation.

Good riddance.

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