iio: chemical: atlas-ezo-sensor: add support for O2 sensor

Add support for the Atlas EZO O2 chemical sensor which required
some refactoring of the driver and parsing of i2c transfer.

Sensor data is converted by the scaling value from percent to
IIO_CONCENTRATION.

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