On other Renesas SoCs, the pin group for the MDIO bus is named "mdio"
instead of "mdc".  Fix the inconsistency, while retaining backwards
compatibility with old DTBs using a pin group alias.

Fixes: b25719eb ("pinctrl: sh-pfc: r8a7795: Add group for AVB MDIO and MII pins")
Fixes: 819fd4bf ("pinctrl: sh-pfc: r8a7795: add EtherAVB support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>