dt-bindings: pinctrl: sh-pfc: Correct SoC family name for R8A7778

R8A7778 is R-Car (not R-Mobile) M1.

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