net: mv88e6xxx: add Clause 45 support

Marvell LinkStreet switches support Clause 45 MDIO on the internal bus.

C45 read or writes require the register address to be written first to
the SMI PHY Data register, and then a special C45 Write Address Register
OP is used on the SMI PHY Register before making a C45 Read Data Register
OP and being able to actually read the register.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>