iio: sps30: add support for serial interface

Sensor has support for both i2c and serial communication interfaces.
Both offer very similar set of features. Minor differences don't impact
overall functionality like doing measurements, etc.

Support for i2c have already been added, this patch adds support
for the latter ie. serial interface.

Signed-off-by: Tomasz Duszynski <tomasz.duszynski@octakon.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>