overlays: Add ugreen-dabboard overlay

This is a simple overlay based on the simple-audio-card and the dmic
codec. It has the speciality that it is configured to use the codec
as a master I2S device. It works for example with the Si468x DAB
receiver on the uGreen DABBoard.

See: https://github.com/raspberrypi/linux/issues/4304



Signed-off-by: Christoph Orth <c.orth@ugreen.eu>