dt-bindings: net: bcmgenet: Add BCM2711 support

The BCM2711 has some modifications to the GENET v5. So add this SoC
specific compatible.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>