dt-bindings: mmc: sdhci-of-dwcmhsc: Add rockchip support

This patch adds rockchip support in sdhci-of-dwcmhsc.yaml

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Link: https://lore.kernel.org/r/1615879102-45919-2-git-send-email-shawn.lin@rock-chips.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>