iio: adc: stm32: add support for triggered buffer mode

STM32 ADC conversions can be launched using hardware triggers.
It can be used to start conversion sequences (group of channels).
Selected channels are select via sequence registers.
Trigger source is selected via 'extsel' (external trigger mux).
Trigger polarity is set to rising edge by default.

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

drivers/iio/adc/Kconfig

@@ -499,6 +499,8 @@ config STM32_ADC_CORE
 	depends on ARCH_STM32 || COMPILE_TEST
 	depends on OF
 	depends on REGULATOR
+	select IIO_BUFFER
+	select IIO_TRIGGERED_BUFFER
 	help
 	  Select this option to enable the core driver for STMicroelectronics
 	  STM32 analog-to-digital converter (ADC).