samples: dac: move HW config to /zephyr,user

Require board overlays to configure the sample in the /zephyr,user
node, and access the device with DEVICE_DT_GET instead of
device_get_binding(). Update the documentation and move existing board
configuration to overlays.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>