pinctrl: sh-pfc: Add PORT_GP_24 helper macro

This follows the style of existing PORT_GP_X macros and
will be used by a follow-up patch for the r8a7791 SoC.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>