dt-bindings: timer: gx6605s SOC timer

Dt-bindings doc for gx6605s SOC's system timer.

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>