ARM: dts: colibri-imx6ull: fix mac address passing

colibri-imx6ull ethernet device is fec2, while the optional secondary
ethernet is fec1, update the ethernet aliases in the .dts file so that
ethaddr is set to fec2 and eth1addr to fec1.

Without this change the ethernet interfaces have a different
mac address between Linux and U-Boot.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>