net: phy: Use PHY MDIO address from DT if available

In case the PHY is fully described in DT, use PHY MDIO address
from DT directly instead of always using auto-detection. This
also fixes the behavior of 'mdio list' in such DT setup, which
now prints the PHY connected to the MAC correctly.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>