pinctrl: sh-pfc: Add PORT_GP_11 helper macro

This follows the style of existion PORT_GP_X macros and
will be used by a follow-up patch for the r8a77990 SoC.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>