hw/arm/aspeed: Add support for loading vbootrom image via "-bios"

hw/arm/aspeed: Add support for loading vbootrom image via "-bios"

Introduce "aspeed_load_vbootrom()" to support loading a virtual boot ROM image
into the vbootrom memory region, using the "-bios" command-line option.

Introduce a new "vbootrom" field in the AspeedMachineClass to indicate whether
a machine supports the virtual boot ROM region.

Set this field to true by default for the AST2700-A0 and AST2700-A1 EVB
machines.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Nabih Estefan <nabihestefan@google.com>
Tested-by: Nabih Estefan <nabihestefan@google.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250424075135.3715128-4-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>