dt-bindings: pinctrl: sh-pfc: Document r8a774b1 PFC support

Document PFC support for the R8A774B1 SoC.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Link: https://lore.kernel.org/r/1568881036-4404-8-git-send-email-biju.das@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>