dt-bindings: iio: stm32-dac: Add support for STM32 DAC

Document STMicroelectronics STM32 DAC (digital-to-analog converter).

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