iio: dac: add support for max5522

Add initial support for DAC max5522.

Tested writing DAC A and B with some values,
from 0 to 1023, measured output voltages, driver works properly.

Additions for max5523/24/25 will follow.

Signed-off-by: Angelo Dureghello <angelo.dureghello@timesys.com>
Link: https://lore.kernel.org/r/20221106165928.223318-1-angelo.dureghello@timesys.com


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