drivers: dac: Allow to set several channels values in one call

[kurddt]

Is your enhancement proposal related to a problem? Please describe.
My application requires that the channels of a same DAC update their values at the same time, so calling dac_write_value is not really satisfying

Describe the solution you'd like
I would like to be able to pass an array of <channel, value> pairs to write to the dac output registers.
We could define a new structure:

struct dac_value {
     u8_t channel;
     u32_t value;
}

and either change the signature of dac_write_value to accept an array of struct dac_value or add an additional function to dac_driver_api

[zephyrbot]

Hi @martinjaeger,

This issue, marked as an Enhancement, was opened a while ago and did not get any traction. It was just assigned to you based on the labels. If you don't consider yourself the right person to address this issue, please re-assing it to the right person.

Please take a moment to review if the issue is still relevant to the project. If it is, please provide feedback and direction on how to move forward. If it is not, has already been addressed, is a duplicate, or is no longer relevant, please close it with a short comment explaining the reason.

@kurddt you are also encouraged to help moving this issue forward by providing additional information and confirming this request/issue is still relevant to you.

Thanks!