ARM: dts: imx6qdl: use enet_clk_ref instead of enet_out for the FEC node

Old imx6q machine code makes RGMII/RMII clock direction decision based on
configuration of "ptp" clock. "enet_out" is not used and make no real
sense, since we can't configure it as output or use it as clock
provider.

Instead of "enet_out" use "enet_clk_ref" which is actual selector to
choose between internal and external clock source:

FEC MAC <---------- enet_clk_ref <--------- SoC PLL
                         \
			  ^------<-> refclock PAD (bi directional)

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