net: phy: mdio-bcm-unimac: Add asp v2.0 support

Add mdio compat string for ASP 2.0 ethernet driver.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
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>