sh-pfc: Make function GPIOs support optional

The target is to get rid of function GPIOs completely. To reach this,
make function GPIOs support optional by skipping the function GPIO chip
registration if no function GPIOS are defined in SoC data.

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>