net: tsec: convert to use DM_MDIO when DM_ETH enabled

For the platforms on which the eTSEC driver uses DM_ETH, convert its
MDIO controller code to also use DM_MDIO.

Note that for handling the TBI PHY (the MAC PCS for SGMII), we still
don't register a udevice for it, since we can drive it locally and there
is no point in doing otherwise.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
[Reworked to fix gazerbeam config]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>