pinctrl: sh-pfc: r8a7795: Add group for AVB MDIO and MII pins

Group the AVB pins into similar groups found in other sh-pfc drivers.
The pins can not be muxed between functions other then AVB but their
drive strength can be controlled.

The group avb_mdc containing ADV_MDC and ADV_MDIO are on other SoCs
called avb_mdio. In pfc-r8a7795 the avb_mdc group already existed and
is in use in DT. Therefore add the ADV_MDIO pin to the existing group
instead of renaming it.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>