hwmon: AD7314 driver (ported from IIO)

Driver for AD7314, ADT7301, and ADT7302, ported from IIO.

Currently dropped power down mode support.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
[guenter.roeck@ericsson.com: Added MODULE_DEVICE_TABLE]
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>