ARM: dts: imx6: Do not hardcode the CLKO clock

Using "IMX6QDL_CLK_CKO" for the clock is easier to read instead of
the hardcoded clock number.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>