iio: frequency: adf4371: Add support for ADF4371 PLL

iio: frequency: adf4371: Add support for ADF4371 PLL

The ADF4371 is a frequency synthesizer with an integrated voltage
controlled oscillator (VCO) for phase-locked loops (PLLs). The ADF4371
has an integrated VCO with a fundamental output frequency ranging from
4000 MHz to 8000 MHz. In addition, the VCO frequency is connected to
divide by 1, 2, 4, 8, 16, 32, or 64 circuits that allows the user to
generate radio frequency (RF) output frequencies as low as 62.5 MHz at
RF8x. A frequency multiplier at RF16x generates from 8 GHz to 16 GHz. A
frequency quadrupler generates frequencies from 16 GHz to 32 GHz at RF32x.
RFAUX8x duplicates the frequency range of RF8x or permits direct access to
the VCO output.

The driver takes the reference input frequency from the device tree and
uses it to calculate and maximize the PFD frequency (frequency of the phase
frequency detector). The PFD frequency is further used to calculate the
timeouts: synthesizer lock, VCO band selection, automatic level
calibration (ALC) and PLL settling time.

This initial driver exposes the attributes for setting the frequency and
enabling/disabling the different adf4371 channels.

Datasheet:
Link: https://www.analog.com/media/en/technical-documentation/data-sheets/adf4371.pdf
Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>