Since commit 0e4a459f ("tracing: Remove unnecessary DEBUG_FS
dependency"), CONFIG_DEBUG_FS is no longer auto-enabled.  This breaks
booting Debian 9, as systemd needs debugfs:

    [FAILED] Failed to mount /sys/kernel/debug.
    See 'systemctl status sys-kernel-debug.mount' for details.
    [DEPEND] Dependency failed for Local File Systems.
    ...
    You are in emergGive root password for maintenance
    (or press Control-D to continue):

Fix this by enabling CONFIG_DEBUG_FS explicitly.

See also commit 18977008 ("ARM: multi_v7_defconfig: Restore
debugfs support").
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20191209101327.26571-1-geert+renesas@glider.be