ARM: imx_v6_v7_defconfig: Select CONFIG_DEBUG_FS

CONFIG_DEBUG_FS is a very useful debug option as it allow us to access key
data such as the clock tree, for example:

mount -t debugfs debugfs /sys/kernel/debug
cat /sys/kernel/debug/clk/clk_summary

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