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

[ Upstream commit 21c0d13e ]

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>
Stable-dep-of: 609bc99a ("dt-bindings: leds: class-multicolor: Fix path to color definitions")
Signed-off-by: Sasha Levin <sashal@kernel.org>