phy dp83867: Make rgmii parameters optional

If you compile without OF_MDIO support in an RGMII configuration, we fail
to configure the dp83867 phy today by writing garbage into its configuration
registers.

On the other hand if you do compile with OF_MDIO and the phy gets loaded via
device tree, you have to have the properties set in the device tree, otherwise
we fail to load the driver and don't even attach the generic phy driver to
the interface anymore.

To make things slightly more consistent, make the rgmii configuration properties
optional and allow a user to omit them in their device tree.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>