pinctrl: sh-pfc: Add PORT_GP_CFG_25() helper macro

They follow the style of the existing PORT_GP_CFG_<n>() macros and
will be used by a follow-up patch for the R8A77980 SoC.

Based on the original (and large) patch by Vladimir Barinov.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>