net: axiemac: use a phandle to reference pcs_phy

In some SGMII use cases where both a fixed link external PHY and the
internal PCS/PMA PHY need to be configured, we should explicitly use a
phandle "pcs-phy" to get the reference to the PCS/PMA PHY. Otherwise, the
driver would use "phy-handle" in the DT as the reference to both the
external and the internal PCS/PMA PHY.

In other cases where the core is connected to a SFP cage, we could still
point phy-handle to the intenal PCS/PMA PHY, and let the driver connect
to the SFP module, if exist, via phylink.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Reviewed-by: Robert Hancock <robert.hancock@calian.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>