dt-bindings: interrupt-controller: C-SKY SMP intc

Dt-bindings doc about C-SKY Multi-processors interrupt controller.

Changelog:
 - Should be: '#interrupt-cells' not 'interrupt-cells'

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Rob Herring <robh@kernel.org>