ARM: dts: r8a7794: add EtherAVB support

Define the generic R8A7794 part of the EtherAVB device node.

Based on the commit 46ece349

 ("ARM: shmobile: r8a7791: add EtherAVB DT
support").

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>