The CLKO is widely used by imx6q board designs to clock audio codec.
Since most codecs accept 24 MHz frequency, let's initially set up CLKO
with OSC24M (cko <-- cko2 <-- osc).  Then those board specific CLKO
setup for audio codec can be removed.

The board dts files also need an update on cko reference in codec node.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>