dt-bindings: iio: ti-dac5571: Add ti,dac121c081

It works much the same as ti,dac7571, so it fits in the "family" even
if the name is a little bit different.

Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/a230b7a8-4146-9483-48ee-0bebdd945731@axentia.se


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