ARM: multi_v7_defconfig: Restore debugfs support

ARM: multi_v7_defconfig: Restore debugfs support

Commit 1575c1a4c3db ("ARM: multi_v7_defconfig: renormalize based on recent
additions") removed explicit enable line for CONFIG_DEBUG_FS, because
that feature has been selected by other enabled options: CONFIG_TRACING,
which were enabled by CONFIG_PERF_EVENTS.

In meantime, commit 110802255eec ("tracing: Remove unnecessary DEBUG_FS
dependency") removed the dependency between CONFIG_DEBUG_FS and
CONFIG_TRACING, so CONFIG_DEBUG_FS is no longer enabled in default builds.

Enable it again explicitly, as debugfs support is essential for various
automated testing tools.

Link: https://lore.kernel.org/r/20191206125112.11006-1-m.szyprowski@samsung.com
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Olof Johansson <olof@lixom.net>