dt-bindings: iio: add vref support to sd modulator

dt-bindings: iio: add vref support to sd modulator

Allow to specify the reference voltage used by the SD modulator.
When the SD modulator is defined as an IIO backend, the reference
voltage can be used to evaluate scaling information of the IIO device.
The reference voltage is not used otherwise.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20240730084640.1307938-7-olivier.moysan@foss.st.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>