cs5535-gpio: apply CS5536 errata workaround for GPIOs

The AMD Geode CS5536 Companion Device Silicon Revision B1 Specification
Update mentions the follow as issue #36:

 "Atomic write transactions to the atomic GPIO High Bank Feature Bit
  registers should only affect the bits selected [...]"

 "after Suspend, an atomic write transaction [...] will clear all
  non-selected bits of the accessed register."

In other words, writing to the high bank for a single GPIO bit will
clear every other GPIO bit (but only sometimes after a suspend).

The workaround described is obvious and simple; do a read-modify-write.
This patch does that, and documents why we're doing it.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>