mdio: Add support for mdio drivers.

Not all devices on an MDIO bus are PHYs. Meaning not all MDIO drivers
are PHY drivers. Add support for generic MDIO drivers.

Change-Id: I65c7c8a497bbac9ef67b3d21c869818a09378e3c
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-Commit: a9049e0c513c4521dbfaa302af8ed08b3366b41f
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Anthony Mah <amah@codeaurora.org>