iio: adc: ti_am335x_adc: make sample delay, open delay, averaging DT parameters

Add optional DT properties to set open delay, sample delay and number
of averages per sample for each adc step. Open delay, sample delay
and averaging are some of the parameters that affect the sampling rate
and accuracy of the sample. Making these parameters configurable via
DT will help in balancing speed vs accuracy.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>