dt-bindings: iio: stm32-adc: add option to set resolution

Add documentation for 'assigned-resolution-bits' dt optional property.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>