Add the EtherAVB pin groups to the R8A7794 PFC driver.

Based on the patches by Mitsuhiro Kimura <mitsuhiro.kimura.kc@renesas.com>.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>