phy: Add MOXA MDIO driver

The MOXA UC-711X hardware(s) has an ethernet controller that seem
to be developed internally. The IC used is "RTL8201CP".

This patch adds an MDIO driver which handles the MII bus.

Signed-off-by: Jonas Jensen <jonas.jensen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>