iio: dac: ti-dac5571: add support for ti,dac121c081

It fits right in as a single 12-bit DAC.

Reviewed-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/a09a49a5-3d01-21f7-02ed-b191d70e7609@axentia.se


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