dt-bindings: pinctrl: sh-pfc: Document r8a774c0 PFC support

Document PFC support for the R8A774C0 SoC.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>