samples: led: lp5569: demo write_channels

Add a simple demonstration of the led_write_channels api on the lp5569
driver sample.
The demonstration simply turns on all of the channels with a single call to
led_write_channels. Then the same is done for turning off the channels.
Thus, it doesn't add much visually, but it shows the usage of the api.

Signed-off-by: Emil Dahl Juhl <emdj@bang-olufsen.dk>