iio: adc: ad7476: Add LTC2314-14 support

The LTC2314-14 is a 14-bit, 4.5Msps, serial sampling A/D converter that draws only
6.2mA from a wide range analog supply adjustable from 2.7V to 5.25V.

Signed-off-by: Dragos Bogdan <dragos.bogdan@analog.com>
Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Link: https://lore.kernel.org/r/20201216083639.89425-1-mircea.caprioru@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>