drm/i915/debugfs: Add perf_limit_reasons in debugfs

Add perf_limit_reasons in debugfs. The upper 16 perf_limit_reasons RW "log"
bits are identical to the lower 16 RO "status" bits except that the "log"
bits remain set until cleared, thereby ensuring the throttling occurrence
is not missed. The clear fop clears the upper 16 "log" bits, the get fop
gets all 32 "log" and "status" bits.

v2: Expand commit message and clarify "log" and "status" bits in
    comment (Rodrigo)

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Tilak Tangudu <tilak.tangudu@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220910143844.1755324-2-ashutosh.dixit@intel.com