ARM: imx_v6_v7_defconfig: enable CONFIG_CPU_FREQ_STAT

It is very useful for user to retrieve cpufreq transtion statistics
and worth to be default enabled.

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