dts: bindings: led: introduce led-node.yaml

This patch moves the label and color-mapping LED property definitions
from led-controller.yaml into led-node.yaml.

This allows to use these properties from a LED node which is not a
child node of a LED controller. This is preparatory work for adding
the leds-group-multicolor binding.

In addition this patch also removes the redundant "label" property
definitions in gpio-leds.yaml and pwm-leds.yaml. It is now included
from led-node.yaml.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>