iio: adc: select triggered buffer for sama5d2 adc

Without the triggered buffer code, we get a link error:

drivers/iio/adc/at91-sama5d2_adc.o: In function `at91_adc_probe':
at91-sama5d2_adc.c:(.text+0x938): undefined reference to `devm_iio_triggered_buffer_setup'

This adds a Kconfig 'select' statement like other ADC
drivers have it already.

Fixes: 5e1a1da0f8c9 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/Kconfig

@@ -158,6 +158,7 @@ config AT91_SAMA5D2_ADC
 	tristate "Atmel AT91 SAMA5D2 ADC"
 	depends on ARCH_AT91 || COMPILE_TEST
 	depends on HAS_IOMEM
+	select IIO_TRIGGERED_BUFFER
 	help
 	  Say yes here to build support for Atmel SAMA5D2 ADC which is
 	  available on SAMA5D2 SoC family.