iio:ad5446: Add support for I2C based DACs

This patch adds support for I2C based single channel DACs to the ad5446
driver. Specifically AD5602, AD5612 and AD5622.

V1: from Lars-Peter Clausen <lars@metafoo.de>
V2: Split the device IDs into two enums and move them to the c file.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>