mmc: dw_mmc: move rockchip related code to a separate file

To support HS200 and UHS-1, we need add a big hunk of code,
as shown in the following patches. So a separate file for
rockchip SOCs is suitable.

Signed-off-by: Addy Ke <addy.ke@rock-chips.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>