ARM: mx25: Add CLKO support

CLKO support is a clock output on mx25 which can output many of the internal
clock sources. It is useful for debugging purpose or also for driving the
audio codec for example.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>