net: mdio-ipq4019: add Clause 45 support

While up-streaming the IPQ4019 driver it was thought that the controller had no Clause 45 support,
but it actually does and its activated by writing a bit to the mode register.

So lets add it as newer SoC-s use the same controller and Clause 45 compliant PHY-s.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>