pinctrl: sh-pfc: r8a7795: Rename EtherAVB "mdc" pin group to "mdio"

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: 30c078de

 ("pinctrl: sh-pfc: r8a7795: Add EtherAVB pins, groups and function")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>