MIPS: mscc: ocelot: mark the phy-mode for internal PHY ports

The ocelot driver was converted to phylink, and that expects a valid
phy_interface_t. Without a phy-mode, of_get_phy_mode returns
PHY_INTERFACE_MODE_NA, which is not ideal because phylink rejects that.

The ocelot driver was patched to treat PHY_INTERFACE_MODE_NA as
PHY_INTERFACE_MODE_INTERNAL to work with the broken DT blobs, but we
should fix the device trees and specify the phy-mode too.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>