mmc: dw_mmc: modify the dt-binding for removing slot-node and supports-highspeed

Almost all SoCs use one slot per host controller.
(Even if controller can support the multiple slot, Recommend to use one slot per host controller.)
Don't use the slot-node and deprecate the "supports-highspeed" property.
Instead, use the cap-mmc/sd-highspeed.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Tushar Behera <trblinux@gmail.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Sachin Kamat <sachin.kamat@samsung.com>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>