ath10k: add support to configure BB timing over wmi

Add wmi configuration cmd to configure base band(BB) power amplifier(PA)
off timing values in hardware. The default PA off timings were fine tuned
to make proper DFS radar detection in QCA reference design. If ODM uses
different PA in their design, then the same default PA off timing values
cannot be used, it requires different settling time to detect radar pulses
very sooner and avoid radar detection problems. In that case it provides
provision to select proper PA off timing values based on the PA hardware used.
The PA component is part of FEM hardware and new device tree entry
"ext-fem-name" is used to indentify the FEM hardware. And this wmi configuration
cmd is enabled via wmi service flag "WMI_SERVICE_BB_TIMING_CONFIG_SUPPORT".

Other way is to apply these values through calibration data, but recalibration
of all boards out there might not be feasible.

This change tested on firmware ver 10.2.4-1.0-00042 in QCA988X chipset.

Signed-off-by: Bhagavathi Perumal S <bperumal@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>