dt-bindings: pinctrl: sh-pfc: Use generic node name

Use the preferred generic node name in the example.

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