Future: Be more flexible with MMC SPL code on future devices.

[kallisti5]

I'm writing this as an OS developer who wants to see riscv64 succeed.
Please, please be consistent with early startup processes.

Here i'm comparing the SiFive Unmatched's MMC format to the MMC of the StarFive Vision2:

The VisionFive 2 is on the left, the Unmatched is on the right. The boards are very similar, minus a few differences:

• The SiFive Unmatched has a dedicated GPT partition UUID for the SPL + u-boot
• The VisionFive has an "FFFF" GPT partition UUID for the SPL + u-boot.

Looking at the JH7110 developer guide, the SPL partition offset is assumed at a fixed offset of 4096 which clashes with the Unmatched.

In future revisions of the BootROM, please allow the dynamic discovery of the MMC SPL by examining the partition table and looking for the offset of a dedicated GPT UUID. (maybe fallback to a fixed address if a valid GPT partition isn't found)