dt-bindings: net: brcm,unimac-mdio: Add asp-v2.0

The ASP 2.0 Ethernet controller uses a brcm unimac.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Justin Chen <justin.chen@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>