iio: chemical: atlas-sensor: add DO-SM module support

Atlas Scientific DO-SM OEM sensor reads disolved oxygen in
a solution. This is reported back as mg/L which maps directly
to ppm and so the IIO_CONCENTRATION channel type can be used.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>