Add EtherAVB pin groups to R8A7790 PFC driver.

Based on original patch by Mitsuhiro Kimura <mitsuhiro.kimura.kc@renesas.com>.
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>