iio: dac: bd79703: Support BD79700 and BD79701

The BD79700 and BD79701 look like almost exact subsets of the BD79703.
The BD79703 contains 6 channels (channels 0 to 5). The BD79700 provides
only 2 channels, matching the BD79703 channels 0 and 1. The BD79701
provides 3 channels (matching BD79703 channels 0, 1, and 2).

Furthermore, the BD79700 and BD79701 do not have separate VFS pin but
use VCC for the full-scale voltage.

Suopport these ICs using the BD79703 driver.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/845aa45974f6fb81c83046368a24a0674e9a8b0e.1743576022.git.mazziesaccount@gmail.com


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