hw/arm/aspeed_ast27x0: Fix unimplemented region overlap with vbootrom

hw/arm/aspeed_ast27x0: Fix unimplemented region overlap with vbootrom

The unimplemented memory region overlaps with the VBootROM address
range, causing incorrect memory layout.

This patch adjusts the size and start address of the unimplemented
region to avoid collision. The IO memory region (ASPEED_DEV_IOMEM) is
now moved to 0x20000 to reserve space for VBootROM at 0x0.

Although the memory range 0x20000 - 0x10000000 is undefined in the
datasheet and should not be required, further testing shows OP-TEE
or U-Boot may access 0x400000 during early boot.
Removing the unimplemented region causes firmware hangs.

To prevent unexpected accesses, retain the region as a safeguard.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250523093144.991408-4-steven_lee@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>