ARM: dts: unuse the slot-node and deprecate the supports-highspeed for dw-mmc in exynos

dw-mmc controller can support multiple slots.
But, there are no use-cases anywhere. So we don't need to support the
slot-node for dw-mmc controller.
And "supports-highspeed" property in dw-mmc is deprecated.
"supports-highspeed" property can be replaced with "cap-sd/mmc-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>
[kgene.kim@samsung.com: rebased exynos5250-snow changes]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>