net: fec: Support phys probed from devicetree and fixed-link

This adds support for specifying the phy to be used with the fec in the
devicetree using the standard phy-handle property and also supports
fixed-link.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>