dt-bindings: leds: lp50xx: fix LED children names

The lp50xx LEDs expects to have single-color LED children with unit
addresses.  This is required by the driver and provided by existing
DTSes.  Fix the binding to match actual usage.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
[This depends on previous schema-fixing patch, so I deleted Fixes; we
don't want to deal with this in stable]
Signed-off-by: Pavel Machek <pavel@ucw.cz>