Documentation: dts: tcs6507: Fix wrong statement about #gpio-cells

Update comment about gpio-cells which says not correctly to be set to 0.
When we want to reference tca6507 gpio #gpio-cells must be set to 2 (it is
already correct in example).
Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

Documentation/devicetree/bindings/leds/tca6507.txt

@@ -8,7 +8,7 @@ Required properties:
 
 Optional properties:
 - gpio-controller: allows lines to be used as output-only GPIOs.
-- #gpio-cells: if present, must be 0.
+- #gpio-cells: if present, must not be 0.
 
 Each led is represented as a sub-node of the ti,tca6507 device.