mmc: dt-bindings: add mmc support to MT7623 SoC

Add the devicetree binding for MT7623 SoC using MT2701 as the fallback.

Cc: devicetree@vger.kernel.org
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>