ARM: mach-imx: imx6ul: remove not optional ethernet refclock overwrite

Ethernet refclock direction is board specific and should be configurable
by devicetree. In fact there are board not working with this defaults,
which will be fixed by separate patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>