ath11k: enable 802.11 power save mode in station mode

To reduce power consumption enable 802.11 power save mode in station mode. This
allows both radio and CPU to sleep more.

Only enable the mode on QCA6390 and WCN6855, it's unknown how other hardware
families support this feature.

To test that power save mode is running run "iw dev wls1 set power_save off",
check there is no NULL Data frame seen by a sniffer. And run "iw dev wls1 set power_save
on" and check there is a NULL Data frame in sniffer.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211108123826.8463-2-kvalo@codeaurora.org