You need modify imx8mp-evk-root.dts to disable SD and enable eMMC. Then the root cell should be able to use eMMC when you booting from eMMC.
Then you need update jailhouse cell configs/arm64/imx8mp-inmate-linux.c to assign emmc sdhc controller to it, not sd sdhc controller to it, the memory map and irq map.
Then you need modify imx8mp-evk-inmate.dts to add sd sdhc in it. since there is no gpio, no regulator as of now, you need force it non-removable and configure regulator ready in root cell, but I guess should not impact.
Needs some jailhouse knowledge to make it work, hope this hepls
Kernel & Virtualization Engineer