dt-bindings:iio:chemical:sensirion,sgp30: Move to trivial-bindings.yaml

The binding for this device and the sgpc3 is very simple so lets
not maintain a seperate document for this one.  Of course we can
always add a document again if the binding becomes more complex in
future.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Andreas Brauchli <andreas.brauchli@sensirion.com>
Link: https://lore.kernel.org/r/20201031182922.743153-2-jic23@kernel.org