dt-bindings:iio:chemical:bosch,bme680: Move to trivial devices

Very simple binding so no need to maintain a separate file.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Sebastien Bourdelin <sebastien.bourdelin@gmail.com>
Cc: Himanshu Jha <himanshujha199640@gmail.com>
Link: https://lore.kernel.org/r/20201031182922.743153-3-jic23@kernel.org