pinctrl: sh-pfc: Add PORT_GP_{10,2[01]} helper macros

This follows the style of existion PORT_GP_X macros and
will be used by a follow-up patch for the r8a77995 SoC.

Extracted from the initial r8a77995 patch in the BSP by Takeshi Kihara.

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