net/cadence/macb: add support for dt phy definition

The macb driver only handle PHY description through platform_data
(macb_platform_data).
Thus, when using dt you cannot define phy properties like phy address or
phy irq pin.

This patch makes use of the of_mdiobus_register to add support for
phy device definition using dt.
A fallback to the autoscan procedure is added in case there is no phy
devices defined in dt.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>