ath11k: add support for spectral scan

spectral scan control interface is exposed through debugfs eentry.
Relayfs is used to collect the spectral data. These interfaces are
similar to ath10k spectral.

spectral debugfs interfaces are below,

echo background > /sys/kernel/debug/ieee80211/phy0/ath11k/spectral_scan_ctl
echo trigger > /sys/kernel/debug/ieee80211/phy0/ath11k/spectral_scan_ctl
iw dev wlan0 scan
echo disable > /sys/kernel/debug/ieee80211/phy0/ath11k/spectral_scan_ctl
cat /sys/kernel/debug/ieee80211/phy0/ath11k/spectral_scan0 > fft_samples.dump

Tested-on: IPQ8074 WLAN.HK.2.1.0.1-01228-QCAHKSWPL_SILICONZ-1

Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1591688014-26441-2-git-send-email-periyasa@codeaurora.org