pinctrl: sh-pfc: Add PORT_GP_CFG_{6|22}() helper macros

They follow the style of the existing PORT_GP_CFG_<n>() macros and
will be used by a follow-up  patch for the R8A77970 SoC.

Based on the original (and large) patch by Daisuke Matsushita
<daisuke.matsushita.ns@hitachi.com>.

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>