iio: si7020: Add devicetree support and trivial bindings

This adds devicetree support for the si7020 iio driver. Since it works
well without requiring any additional property, its compatible string is
added to the trivial i2c devices bindings list.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>