pinctrl: sh-pfc: r8a7796: Add group for AVB MDIO and MII pins

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

The group avb_mdc containing ADV_MDC and ADV_MDIO is called avb_mdio on
R-Car Gen2 SoCs.  In pfc-r8a7796 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.

Based on commit b25719eb ("pinctrl: sh-pfc: r8a7795: Add group
for AVB MDIO and MII pins").

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