Documentation: dt: iio: stm32-adc: optional dma support

STM32 ADC can use dma. Add dt documentation for optional dma support.

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