dt-bindings: leds: Optional multi-led unit address

The unit address does not make sense in all cases the multi-led node is
used, e.g. for the upcoming PWM multi-color LED driver.

Signed-off-by: Sven Schwermer <sven.schwermer@disruptive-technologies.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>