iwlwifi: mvm: add dbg_time_point to debugfs

We forgot to link this to debugfs, so the code is all dead.
Add it for real.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/iwlwifi.20211219110000.d0f314101410.I7357c01179c35621686265d4da4a64d2333a5f1a@changeid