net: designware: add DM_MDIO support

Add support for DM_MDIO to connect to PHY and expose a MDIO device for the
internal MDIO bus in order to dynamically connect to MDIO PHYs with DT
with eventual MDIO muxes in between.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>