pinctrl: sh-pfc: Share/reuse same PORT_GP_x() macros

Many SoC needs each PORT_GP_x() macros, but we can share/reuse
same one.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>