iio:ad7793: Add support for the ad7798 and ad7799

The ad7798 and ad7799 are similar to the ad7792 and ad7793 but are missing some
features like the temperature sensor, being able to use an external clocksource
and a few other things. This patch adds a new 'flags' fields to the chip_info
struct which allows to specify which features a certain chip variant supports.
The setup code will then ignore any platform data fields which are related to
non supported features.

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