dt-bindings: timer: C-SKY Multi-processor timer

Dt-bingdings doc for C-SKY SMP system setting.

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