ARM: imx_v6_v7_defconfig: Explicitly restore CONFIG_DEBUG_FS

This is currently off and that's not desirable: default imx config is
meant to be generally useful for development and debugging.

Running git bisect between v5.4 and v5.5-rc1 finds this started from
commit 0e4a459f ("tracing: Remove unnecessary DEBUG_FS dependency")

Explicit CONFIG_DEBUG_FS=y was earlier removed by
commit c29d541f ("ARM: imx_v6_v7_defconfig: Remove unneeded options")

A very similar fix was required before:
commit 7e9eb626

 ("ARM: imx_v6_v7_defconfig: Explicitly restore CONFIG_DEBUG_FS")

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>