ath11k: Add support for ring backpressure stats

Add support for collecting and dumping the ring backpressure
stats via debugfs. Stats are dumped only if events are
received for the specific ring.

Below command can be used to obtain these stats as part of soc dp stats.
cat /sys/kernel/debug/ath11k/ipq8074/soc_dp_stats

 Sample Output - When No stats available:

 Backpressure Stats
 ==================
 No Ring Backpressure stats received

 Sample Output -  When ring bp stats available for specific ring

 Backpressure Stats
 ==================
 Ring: REO2SW1_RING
 count: 1
 hp: 2
 tp: 2
 seen before: 4ms

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.1.0.1-01213-QCAHKSWPL_SILICONZ-1

Signed-off-by: Sriram R <srirrama@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1591768308-32005-3-git-send-email-srirrama@codeaurora.org