perf/arm-cmn: Make arm_cmn_debugfs static

Indeed our debugfs directory is driver-internal so should be static.

Link: https://lore.kernel.org/r/202202030812.II1K2ZXf-lkp@intel.com


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/ca9248caaae69b5134f69e085fe78905dfe74378.1643911278.git.robin.murphy@arm.com


Signed-off-by: Will Deacon <will@kernel.org>