ARM: dts: imx6q-cm-fx6: fix fec pinctrl

According to the schematics of CompuLab's sbc-fx6 baseboard and the
vendor devicetree GPIO_16 is *not* muxed to ENET_REF_CLK but to SPDIF_IN.

Remove the wrong pinctrl setting.

Fixes: 682d055e ("ARM: dts: Add initial support for cm-fx6.")
Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>