pinctrl: sh-pfc: r8a7796: 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: 41397032 ("pinctrl: sh-pfc: r8a7796: Add group for AVB MDIO and MII pins")
Fixes: 9c99a63e ("pinctrl: sh-pfc: r8a7796: Add EtherAVB pins, groups and functions")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>