net: phy: mdio-octeon: Cleanup module loading dependencies

To ensure that the octeon MDIO driver has been loaded, the Cavium
ethernet drivers reference a dummy symbol in the MDIO driver. This
forces it to be loaded first. And this symbol has not been cleanly
implemented, resulting in warnings when build W=1 C=1.

Since device tree is being used, and a phandle points to the PHY on
the MDIO bus, we can make use of deferred probing. If the PHY fails to
connect, it should be because the MDIO bus driver has not loaded
yet. Return -EPROBE_DEFER so it will be tried again later.

Additionally, add a MODULE_SOFTDEP() to give user space a hint as to
what order it should load the modules.

v2:
s/octoen/octeon/
Add MODULE_SOFTDEP()

Cc: Sunil Goutham <sgoutham@marvell.com>
Cc: Robert Richter <rrichter@marvell.com>
Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>