ARM: shmobile: r8a7790: add EtherAVB DT support

Define the generic R8A7790 part of the EtherAVB device node.

Based on original patch by Mitsuhiro Kimura <mitsuhiro.kimura.kc@renesas.com>.

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