pinctrl: sh-pfc: r8a7795: Make PORT_GP_x() macros consistent

On r8a7795, PORT_GP_x() is a macro for defining GPIOs 0..x.
In all other sh-pfc code, PORT_GP_x() is a macro for defining GPIOs
0..(x-1).

Make the r8a7795 macro definitions consistent with the rest of the
sh-pfc codebase.

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>