Documentation: ABI: testing: add common ABI file for iio/frequency

Add ABI documentation file for PLL/DDS devices with frequency_resolution
sysfs entry attribute used by both ADF4350 and ADF41513.

Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>