Define the generic R8A7792 part of the EtherAVB device node.

Based on the commit 89aac8af ("ARM: dts: r8a7794: add EtherAVB support").
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>