pc-bios: Add AST27x0 vBootrom

pc-bios: Add AST27x0 vBootrom

The boot ROM is a minimal implementation designed to load an AST27x0 boot image.
Its source code is available at:
https://github.com/google/vbootrom
Commit id: d6e3386709b3e49322a94ffadc2aaab9944ab77b
Build Information:
```
Build Date : Apr 29 2025 01:23:18
FW Version : git-d6e3386
```

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Nabih Estefan <nabihestefan@google.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: Nabih Estefan <nabihestefan@google.com>
Link: https://lore.kernel.org/qemu-devel/20250429062822.1184920-2-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>