device-types: Update meson-g12b-a311d-khadas-vim3 for mainline U-Boot
The VIM3 is properly supported in mainline U-Boot, so we should use mainline U-Boot in LAVA. Remove the custom device-type configuration for vendor U-Boot and update the memory offsets for the kernel artifacts to align with mainline U-Boot.
[proposed upstream at https://gitlab.com/lava/lava/-/merge_requests/2471]