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

Introduce new compatible to support STM32F4 DAC (Digital-To-Analog
converter) variant.
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/dac/st,stm32-dac.txt

@@ -10,7 +10,9 @@ current.
 Contents of a stm32 dac root node:
 -----------------------------------
 Required properties:
-- compatible: Must be "st,stm32h7-dac-core".
+- compatible: Should be one of:
+  "st,stm32f4-dac-core"
+  "st,stm32h7-dac-core"
 - reg: Offset and length of the device's register set.
 - clocks: Must contain an entry for pclk (which feeds the peripheral bus
   interface)