net: phy: Support "internal" PHY interface

Now that the Device Tree binding has been updated, update the PHY
library phy_interface_t and phy_modes to support the "internal" PHY
interface type.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>