FB: sa11x0: fix shannon GPSR/GPCR accesses

The GPIO set and clear registers should only be written, rather than
read, modified, and written.  A read-modify-write will have undesired
side effects.

Acked-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>