dt-bindings: leds: Update devicetree documents for ID_RGB

Update the leds/common.yaml to indicate that the max color ID is 9.
Reflect the same change in the leds-class-multicolor.yaml

Reported-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20201016115703.30184-1-dmurphy@ti.com
Signed-off-by: Rob Herring <robh@kernel.org>