pinctrl: sh-pfc: r8a7794: Use PINMUX_SINGLE() instead of raw PINMUX_DATA()

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>