mmc: dt-bindings: add MMC support to JZ4740 SoC

Add the devicetree binding for JZ4740/JZ4780 SoC MMC/SD controller.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.co.uk>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>