dt-bindings: net: renesas,etheravb: Document RZ/G3S support

Document Ethernet RZ/G3S support. Ethernet IP is similar to the one
available on RZ/G2L devices.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>