arm64: dts: imx8mp: Enable HS400-ES

The SDHC controller in the imx8mp has the same controller
as the imx8mm which supports HS400-ES. Change the compatible
fallback to imx8mm to enable it, but keep the imx7d-usdhc
to prevent breaking backwards compatibility.

Signed-off-by: Adam Ford <aford173@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>