drivers: sensor: dacx3608: add broadcast register for synchronized output

The dacx3608 line supports a broadcast register so all configured channels
can output a singular value, simultaneously. This drastically reduces I2C
overhead when using multi-channel control. An API addition was necessary
to support a global broadcast channel number. The API addition does not
break the write_value() implementation for other DAC drivers in the repo.
This change is based on an out-of-tree driver developed internally to
handle this use case.

Alternative to the API change, could be a KConfig option or device tree
entry. Also, no support for the Broadcast channel was added to the
channel_setup() implementation - this may or may not be confusing. I
believe it makes sense to maintain explicit setup calls for each channel
intended to be configured.

Signed-off-by: Ben Booher <benbooher@pull-repo.com>