dt-bindings: iio: stm32-adc: add max clock rate property

Add optional dt property to tune maximum desired analog clock rate.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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

@@ -53,6 +53,8 @@ Optional properties:
   analog input switches on stm32mp1.
 - st,syscfg: Phandle to system configuration controller. It can be used to
   control the analog circuitry on stm32mp1.
+- st,max-clk-rate-hz: Allow to specify desired max clock rate used by analog
+  circuitry.
 
 Contents of a stm32 adc child node:
 -----------------------------------