net: bgmac: support MDIO described in DT

Check ethernet controller DT node for "mdio" subnode and use it with
of_mdiobus_register() when present. That allows specifying MDIO and its
PHY devices in a standard DT based way.

This is required for BCM53573 SoC support. That family is sometimes
called Northstar (by marketing?) but is quite different from it. It uses
different CPU(s) and many different hw blocks.

One of shared blocks in BCM53573 is Ethernet controller. Switch however
is not SRAB accessible (as it Northstar) but is MDIO attached.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>