Define the generic R8A7791 part of the EtherAVB device node.

Based on the commit f25d6b97 ("ARM: shmobile: r8a7790: add EtherAVB DT
support").
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>