iio: chemical: sps30: add support for self cleaning

Self cleaning is especially useful in cases where sensor undergoes
frequent power on/off cycles. In such scenarios it is recommended to
turn self cleaning at least once per week in order to maintain reliable
measurements.

Self cleaning is activated by writing 1 to a dedicated attribute.
Internal fan accelerates to its maximum speed and keeps spinning
for about 10 seconds blowing out accumulated dust.

Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
Tested-by: Andreas Brauchli <andreas.brauchli@sensirion.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>