SAUL: missing DAC support

[jnohlgard]

Add periph/dac support to SAUL.

[jnohlgard]

I will likely work on implementing this right after #4430, #4431, #4860, #4863, #4814 are merged.

[miri64]

They are ;-)

[OlegHahm]

Still willing?

[jnohlgard]

I did not have time to fix this, but it is still a request to add the functionality to SAUL.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.