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>