dt-bindings: mmc: Convert MMC Card binding to a schema

MMC Cards can have an optional Device Tree binding to add
non-discoverable properties.

Now that we have the DT validation in place, let's convert the device
tree bindings for that driver over to a YAML schema.

Some of these properties were already described in the MMC controller
binding, even though they are not generic and do not apply to any
device, so we took the occasion to fix this.

Cc: linux-mmc@vger.kernel.org
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210901091852.479202-31-maxime@cerno.tech
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>