staging:iio:ad7793: Rework platform data

Currently the platform data for the ad7793 consist just out of the raw default
register settings. This has some downsides, for one we actually don't want to
make all bits configurable and secondly not all register settings are actually
valid. This patch exposes all the options which should be configurable via
platform data as induvidual platform data struct fields. This also allows us to
document the different settings via proper kernel doc.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>