ARM: mxs: defconfig: Enable CONFIG_COMMON_CLK_DEBUG

With the common clock framework in place, it is useful to enable
CONFIG_COMMON_CLK_DEBUG option, in order to easily inspect the clock tree.

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