hw/arm/aspeed: Move aspeed_load_vbootrom to common SoC code

Move the vbootrom loader helper into common SoC code so it can be reused
by all ASPEED boards, and decouple the API from AspeedMachineState.

Specifically:
- Move aspeed_load_vbootrom() to hw/arm/aspeed_soc_common.c and
declare it in include/hw/arm/aspeed_soc.h.
- Change the helper’s signature to take AspeedSoCState * instead of
AspeedMachineState *.
- Update aspeed_machine_init() call sites accordingly.

No functional change.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250925050535.2657256-5-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
3 files changed