net: phy: mdio-bcm-unimac: Add asp v2.2 support

Add mdio compat string for ASP 2.0 ethernet driver.

Signed-off-by: Justin Chen <justin.chen@broadcom.com>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>