ARM: dts: imx6qdl-colibri: add phy to fec

Add the phy-node and mdio bus to the fec-node, represented as is on
hardware.
This commit includes micrel,led-mode that is set to the default
value, prepared for someone who wants to change this.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>