ARM: exynos_defconfig: Restore debugfs support

ARM: exynos_defconfig: Restore debugfs support

Commit 490800e90385 ("ARM: exynos_defconfig: Trim and reorganize with
savedefconfig") removed explicit enable line for CONFIG_DEBUG_FS, because
that feature has been selected by other enabled options: CONFIG_TRACING,
which in turn had been selected by CONFIG_PERF_EVENTS and
CONFIG_PROVE_LOCKING.

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.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>