Documentation: synopsys-dw-mshc: add binding for resets

Add resets property to synopsys-dw-mshc bindings. It is intended to
represent the hardware reset signal present internally in some host
controller IC designs.

See Documentation/devicetree/bindings/reset/reset.txt for details.

Signed-off-by: Guodong Xu <guodong.xu@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>