dt-bindings: reset: Add HI3670 reset controller binding

HI3670 SoC is architecturally same as the HI3660 SoC. Hence, the same
driver is reused for HI3670 SoC and the binding is documented here which
uses the fallback approach.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>