pinctrl: sh-pfc: Add PORT_GP_27 helper macro

This follows the style of the existing PORT_GP_X macros, and will be
used by a follow-up patch for the r8a7778 SoC.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>