dt-bindings: net: ravb: Add support for r8a774b1 SoC

Document RZ/G2N (R8A774B1) SoC bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>