sh-pfc: Skip gpiochip registration when no GPIO resource is found
Boards/platforms that register dedicated GPIO devices will not supply a
memory resource for GPIOs. Try to locate the GPIO memory resource at
initialization time, and skip registration of the gpiochip if the
resource can't be found.
This is a temporary modification to ease the transition to separate GPIO
drivers. It should be reverted when all boards and platforms will have
been moved.