ARM: socfpga_defconfig: Add back DEBUG_FS

Commit 0e4a459f ("tracing: Remove unnecessary DEBUG_FS dependency")
removed select for DEBUG_FS but we still need it for development purposes.

Fixes: 0e4a459f

 ("tracing: Remove unnecessary DEBUG_FS dependency")
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>