starfive: visionfive2: add mmc0 and nvme boot targets

boot from SDIO3.0 (mmc sdcard) first if it is plugged.
If mmc is not plugged try to boot from emmc if it is plugged.
If emmc is not plugged then try to boot from nvme.

Signed-off-by: Milan P. Stanić <mps@arvanta.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>