net: nixge: Make mdio child node optional

Make MDIO child optional and only instantiate the
MDIO bus if the child is actually present.

There are currently no (in-tree) users of this
binding; all (out-of-tree) users use overlays that
get shipped together with the FPGA images that contain
the IP.

This will significantly increase maintainabilty
of future revisions of this IP.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>