dt-bindings: iio: chemical: sps30: update binding with serial example

sps30 has gained support for serial communication so add example to the
binding file. While at it remove reg property from list of required
properties because it's no-op in case of serial communication.

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