dt-bindings: iio: stm32-dac: add support for STM32F4

Introduce new compatible to support STM32F4 DAC (Digital-To-Analog
converter) variant.

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