sh-pfc: Make GPIO support optional

When implemented as a separate IP block, GPIOs should be handled by a
separate driver. To make this possible GPIO support needs to be optional
in the sh-pfc driver.

If no GPIO data registers are supplied in the SoC information structure
skip registration of the gpiochip.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>