dt-bindings: Add ROHM BD7970x variants

dt-bindings: Add ROHM BD7970x variants

The ROHM BD79700, BD79701 and BD79702 are subsets of the BD79703 DAC.
The main difference is the number of the channels. BD79703 has 6
channels.

The BD79702 has 4, BD79701 3 and BD79700 2 channels. Additionally, the
BD79700 and BD79701 do not have separate Vfs pin but use the Vcc also
for the full-scale voltage.

Add properties for the BD79700, BD79701 and BD79702.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/0a114565e4de52bf8f98c4f9d17943e5148b0112.1743576022.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>