dt-bindings: mmc: Add HI3670 MMC controller binding

HI3670 SoC is architecturally same as the HI3660 SoC. Hence, the same
K3 specific designware driver is reused for HI3670 SoC and the binding
is documented with fallback approach for compatible property.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>