iio:adc: Driver for Linear Technology LTC2497 ADC

This patch adds support for the Analog Devices / Linear Technology
LTC2497 ADCs. The LTC2497 is a 16-channel (eight differential),
16-bit, high precision, delta-sigma ADC with an automatic, differential,
input current cancellation front end and a 2-wire, I2C interface.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>