iio: adc: rockchip_saradc: select IIO_TRIGGERED_BUFFER

iio: adc: rockchip_saradc: select IIO_TRIGGERED_BUFFER

The kernel fails to compile due to undefined reference to
devm_iio_triggered_buffer_setup() if IIO_TRIGGERED_BUFFER is not
enabled. The original patch [1] had this dependency. But somehow it
didn't make it into the kernel tree. Re-add it.

[1] https://lore.kernel.org/lkml/20200623233011.2319035-3-heiko@sntech.de/

Fixes: 4e130dc7b413 ("iio: adc: rockchip_saradc: Add support iio buffers")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>