ath10k: add spectral scan feature

Adds the spectral scan feature for ath10k. The spectral scan is triggered by
configuring a mode through a debugfs control file. Samples can be gathered via
another relay debugfs file.

Essentially, to try it out:

ip link set dev wlan0 up
echo background > /sys/kernel/debug/ieee80211/phy0/ath10k/spectral_scan_ctl
echo trigger > /sys/kernel/debug/ieee80211/phy0/ath10k/spectral_scan_ctl
iw dev wlan0 scan
echo disable > /sys/kernel/debug/ieee80211/phy0/ath10k/spectral_scan_ctl
cat /sys/kernel/debug/ieee80211/phy0/ath10k/spectral_scan0 > samples

This feature is still experimental. Based on the original RFC patch of
Sven Eckelmann.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>