iio: dac: ad5791: Add support for controlling RBUF via devicetree

This patch adds support for an external amplifier to be connected in a
gain of two configuration.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231129-ad5791-michael-stuff-v3-2-48e192b00909@analog.com


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