dt-bindings: mmc: sdhci-am654: Convert sdhci-am654 controller documentation to json schema

Convert sdhci-am654 documentation to yaml format. The new file
sdhci-am654.yaml will inherit from mmc-controller.yaml.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Link: https://lore.kernel.org/r/20200923105206.7988-2-faiz_abbas@ti.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>