dt-bindings: mmc: add bindings for hi3798cv200-dw-mshc

Hisilicon hi3798cv200 SoC extends the dw-mshc controller
for additional clock control. Add device tree bindings for
hi3798cv200-dw-mshc.

Signed-off-by: tianshuliang <tianshuliang@hisilicon.com>
Signed-off-by: Jiancheng Xue <xuejiancheng@hisilicon.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>