Add mdio command for new PHY infrastructure

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>