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: 5e1a1da0 ("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>