staging:iio:ad7192: Add support for the AD7193

The AD7193 is mostly register map compatible to the AD7192. The main
difference is that it has 8 instead of 4 inputs, so the way the input
channels are selected is slightly different.

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