ath10k: add quiet mode support for QCA6174/QCA9377

To enable thermal throttling for QCA6174 and QCA9377,
implement gen_pdev_set_quiet_mode for them.
With this change, quiet period for QCA6174/QCA9377 can
be also set/get via debugfs.
Usage:
To set quiet period:
 echo <period> > /sys/kernel/debug/ieee80211/phyX/ath10k/quiet_period
To get the current quiet period:
 cat /sys/kernel/debug/ieee80211/phyX/ath10k/quiet_period

This change has been verified with
QCA6174 hw3.2(fw: WLAN_RM.4.4.1-00102-QCARMSWP-1).

Signed-off-by: Yu Wang <yyuwang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>