arm64: dts: hi6220: add resets property into dwmmc nodes

Add resets property into dwmmc_0, dwmmc_1 and dwmmc_2 for hi6220

Code and documentation to this property were confirmed by maintainers.
See:
[1] https://patchwork.kernel.org/patch/9276607/
[2] https://patchwork.kernel.org/patch/8487151/
[3] https://lkml.org/lkml/2016/8/12/91



cc: Jaehoon Chung <jh80.chung@samsung.com>
cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>