pinctrl: sh-pfc: Stop including <linux/platform_data/gpio-rcar.h>

This header file will be removed soon.

Copy the helper macro RCAR_GP_PIN(), which is used by the pinctrl
drivers only, to sh_pfc.h, and drop the #include.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>