ARM: imx6q: remove board specific CLKO setup

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>