dt-bindings: mmc: renesas,mmcif: Convert to json-schema

Convert the Renesas Multi Media Card Interface (MMCIF) Device Tree
binding documentation to json-schema.

Document missing properties.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/04b97315fed0f4f512356b68f9f5bb6ed7adc41f.1620648698.git.geert+renesas@glider.be


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>