dt-bindings: mmc: Add YAML schemas for the generic MMC options

The MMC controllers have a bunch of generic options that are needed in a
device tree. Add a YAML schemas for those.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>