CHROMIUM: wireless: Add summary for transmit latency

Add debugfs entry for transmit latency statistics summary, which shows
the cumulative percentage of transmit latency for all ACs that has
actvity.
Write 0 to this entry will clear counters for all ACs.

commit 6a72431e39b9 ("CHROMIUM: wireless-4.2: Add summary for transmit latency")
https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/541626



BUG=b:122726016
TEST=cat /sys/kernel/debug/ieee80211/phyX/ath10k/stats

Cq-Depend: chromium:1677132
Change-Id: I278e057de694dbfc74bd9bbbef97884d1967a131
Signed-off-by: Ratheesh S P <ratheeshs@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1676934


Tested-by: Ratheesh Selvakumar Pappa <ratheeshs@google.com>
Commit-Ready: Ratheesh Selvakumar Pappa <ratheeshs@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Danny Mitzel <mitzel@google.com>
Reviewed-by: Kan Yan <kyan@chromium.org>