dt-bindings: net: Brcm ASP 2.0 Ethernet controller

Add a binding document for the Broadcom ASP 2.0 Ethernet
controller.

Reviewed-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>