dwc: Support phy mode MII

Some board use dwc phy in MII mode, so do not fail to attach if this is
the case.
Only rockchip code uses the phy mode to program some custom syscon register.

PR: 260848
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit da6252a6a099e6253207b69960e6762ce3cca0f8)