iio: Add LTC2471/LTC2473 driver

The LTC2741 and LTC2473 are single voltage ADC chips. The LTC2473
is similar to the LTC2471 but outputs a signed differential value.

Datasheet:
  http://cds.linear.com/docs/en/datasheet/24713fb.pdf



Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>