ARM: dts: imx6ul: set enet_clk_ref to CLK_ENETx_REF_SEL

IMX6UL_CLK_ENETx_REF is behind of CLK_ENETx_REF_SEL:

FEC MAC <---------- CLK_ENETx_REF_SEL <--------- CLK_ENETx_REF
		       \
		        ^------<-> CLK_ENETx_REF_PAD

We should point to the clock selector instead. So, we will be able to
use external clock source from CLK_ENETx_REF_PAD as well.

At same time, remove enet_out clk. It is using always the same clock as
enet_clk_ref and do not help to solve any challenges of this HW.

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