dt-bindings: Document the hi3660 reset bindings

Add DT bindings documentation for hi3660 SoC reset controller.

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>