Split ad_pulsar.c into two separate drivers?

[dlech]

While working on #2280, I noticed that the drivers/iio/adc/ad_pulsar.c currently supports two quite different types of chips. This makes the driver harder to understand and more prone to bugs. It seems like it would be better to split this up into two drivers.

The first type of chip are the multichannel ADCs like AD7682. These chips also have configuration registers. Chips like AD7685 on the, on the other hand, are single channel and don't have configuration registers. The AD7682-like chips can be configured to be single-ended or differential while the AD7685-like chips are always differential. Also, it is not clear which of the 3 conversion timing modes is being used on the AD7682-like chips, but it looks like it could be "read/write during conversion (RDC)" which is different from the AD7685-like chips as well.

As it is, it looks like there are quite a few places in this driver where it is attempting to write to configuration registers whether or not the ADC chip actually has any.