dt-bindings: reset: correct vendor prefix hisi to hisilicon

The vendor prefix of "Hisilicon Limited" is "hisilicon", it is clearly
stated in "vendor-prefixes.yaml".

Fixes: 836e2354 ("dt-bindings: Document the hi3660 reset bindings")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Zhangfei Gao <zhangfei.gao@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>