net: Add eth phy generic driver for shared MDIO

For dual ethernet controllers, the HW design may connect ETH phys to
one MDIO ports. So two different ethernet drivers have to share MDIO bus.
Since two ethernet drivers are independent, we can't ensure their probe
order.

To resolve this problem, introduce an eth phy generic driver and uclass.

After eth-uclass binds, we search the mdio node and binds the phy node
with the eth-phy-generic driver.

When one eth driver get its phy device, the parent of phy device will
probe prior than phy device. So this ensure the eth driver ownes the
MDIO bus will be probed before using its MDIO.

Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>