dt: bindings: lp8860: Update DT label binding

Update the lp8860 label binding to the LED
standard as documented in

Documentation/devicetree/bindings/leds/common.txt

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>