net/phy: fix mdio-octeon dependency and build

Fix build errors by making this driver depend on OF_MDIO, like
several other similar drivers do.

drivers/built-in.o: In function `octeon_mdiobus_remove':
mdio-octeon.c:(.text+0x196ee0): undefined reference to `mdiobus_unregister'
mdio-octeon.c:(.text+0x196ee8): undefined reference to `mdiobus_free'
drivers/built-in.o: In function `octeon_mdiobus_probe':
mdio-octeon.c:(.text+0x196f1d): undefined reference to `devm_mdiobus_alloc_size'
mdio-octeon.c:(.text+0x196ffe): undefined reference to `of_mdiobus_register'
mdio-octeon.c:(.text+0x197010): undefined reference to `mdiobus_free'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc:	Andrew Lunn <andrew@lunn.ch>
Cc:	Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>