iio: frequency: adf4371: Add support for ADF4372 PLL

The ADF4372 is part of the same family with ADF4371, the main difference
is that it has only 3 channels instead of 4, as the frequency quadrupler
is missing. As a result, the ADF4372 allows frequencies from 62.5 MHz to
16 GHz to be generated.

Datasheet:
Link: https://www.analog.com/media/en/technical-documentation/data-sheets/adf4372.pdf



Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>