iio: dac: bd79703: Support ROHM BD79702

iio: dac: bd79703: Support ROHM BD79702

The ROHM BD79702 is similar to the BD79703, except that it has only 4
channels whereas BD79703 has 6 channels. The channel 'addresses' of the
first two channels (used to identify the channel when data is read over
SPI) are same for both ICs. The next two channels of the BD79702 have
same addresses as the last two channels of the BD79703. This means the
BD79702 channel addresses do not follow the channel numbers with a
constant offset. Thus, we need to specify the addresses separately,
instead of directly deriving them from the channel number with a
constant offset.

It's worth noting that the data-sheet describes the BD79702 as a device
having channels 1,2,5 and 6. The driver however represents channels
0,1,2,3 to the users - with no gaps in the numbering - which may be more
familiar view for the application software.

Support ROHM BD79702 DAC.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/0ba243a63115dd4af03ebf9656c65b8c259a3e34.1743576022.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>