iio:ad7793: Add support for the ad7796 and ad7797

The ad7796 and ad7797 are similar to the ad7792 and ad7793 but only have a
single differential input instead of two. Also some other features are missing
like the programmable gain amplifier and also not all sampling frequencies
supported by the ad7792/ad7793 are supported by the ad7796/ad7797. This patch
adds new feature flags for the features not present in the ad7796/ad7797. The
patch also adds a struct iio_info field to the chip_info struct, this becomes
necessary since the ad7796/ad7797 needs a special set of sysfs attributes.

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