ravb: document R8A77970 bindings

R-Car V3M (R8A77970) SoC also has the R-Car gen3 compatible EtherAVB
device, so document  the SoC specific bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>