FCC long pulse radar (type 5) requires pulses to be
checked for chirping. This patch implements chirp
detection based on the FFT data provided for long
pulses.

A chirp is detected when a set of criteria defined
by FCC pulse characteristics is met, including
* have at least 4 FFT samples
* max_bin index moves equidistantly between samples
* the gradient is within defined range

The chirp detection has been tested with reference
radar generating devices and proved to work reliably.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>