leds: lp8860: Update the dt parsing for LED labeling

Update the DT parsing for the label node so that
the label is retrieved from the device child as
opposed to being part of the parent.

This will align this driver with the LED
binding documentation

Documentation/devicetree/bindings/leds/common.txt

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>