Add mdio command for new PHY infrastructure

The new mdio command doesn't have all of the features of the mii
command, but it provides the necessary read/write primitives, and allows
users to interact with 10G PHYs, and other PHYs which use Clause 45 of
802.3.  This means that the mdio command requires a "Device Address"
argument, though for clause 22 PHYs, the argument can be "-".

Signed-off-by: Andy Fleming <afleming@freescale.com>
Acked-by: Detlev Zundel <dzu@denx.de>