net: tsec: Add fixed-link PHY support

The info of fixed-link PHY is described in DT node instead of
getting from MII, so detect the fixed-link PHY DT node first,
if it doesn't exist then probe the MII.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>