iio: adc: ltc2485: add support for Linear Technology LTC2485 ADC

Adds basic support for the LTC2485 ADC - a delta-sigma analog-to-digital
converter with an I2C interface that operates in single shot conversion
mode.

The driver supports an on board 5V reference and the power-on default
configuration which rejects both 50hz & 60hz line frequencies and
operates in 1x speed mode.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>