drivers:iio:dac:ad5766.c: Add trigger buffer

This chip is able to generate waveform and using an
with the output trigger buffer will be easy to generate one.

Signed-off-by: Mihail Chindris <mihail.chindris@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20211007080035.2531-7-mihail.chindris@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>