pinctrl: sh-pfc: Confine legacy function GPIOs to SH

Legacy function GPIOs are no longer used on ARM since commit
a27c5cd1 ("sh-pfc: sh73a0: Remove function GPIOs").
Extract its setup code into a separate function, and make all function
GPIO related code and data depend on CONFIG_SUPERH.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>