powerpc/86xx: Basic GPIO support for GE Fanuc SBC610

Basic support for the GPIO available on the SBC610 VPX Single Board Computer
from GE Fanuc (PowerPC MPC8641D).

This patch adds basic support for the GPIO in the devices I/O FPGA, the GPIO
functionality is exposed through the AFIX pins on the backplane, unless used
by an AFIX card.

This code currently does not support switching between totem-pole and
open-drain outputs (when used as outputs, GPIOs default to totem-pole).
The interrupt capabilites of the GPIO lines is also not currently supported.

Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>