gpio calls don't need i/o barriers

Clarify that drivers using the GPIO operations don't need to issue io
barrier instructions themselves.  Previously this wasn't clear, and at
least one platform assumed otherwise (and would thus break various
otherwise-portable drivers which don't issue barriers).
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/gpio.txt

@@ -75,6 +75,9 @@ using the include file:
 If you stick to this convention then it'll be easier for other developers to
 see what your code is doing, and help maintain it.
 
+Note that these operations include I/O barriers on platforms which need to
+use them; drivers don't need to add them explicitly.
+
 
 Identifying GPIOs
 -----------------