On other Renesas SoCs, the pin group for the MDIO bus is named "mdio"
instead of "mdc".  Fix the inconsistency, now the pinctrl drivers for
R-Car H3, M3-W, and M3-N have gained support for the traditional pin
group name.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>