dt-bindings: mmc: sdhci-of-dwcmhsc: Convert to yaml file

This patch converts sdhci-of-dwcmshc.txt to sdhci-of-dwcmshc.yaml

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1615879102-45919-1-git-send-email-shawn.lin@rock-chips.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>