dt-bindings: backlight: kinetic,ktz8866: Add missing type for "current-num-sinks"

"current-num-sinks" is missing a type, add it.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230613201022.2823392-1-robh@kernel.org


Signed-off-by: Lee Jones <lee@kernel.org>