hw/ssi/aspeed_smc: Allow 64-bit wide flash accesses

hw/ssi/aspeed_smc: Allow 64-bit wide flash accesses

cde3247651dc998da5dc1005148302a90d72f21f fixed atomicity for LDRD, which
ends up making accesses 64-bits wide. However, the AST2600 bootloader
can sometimes compile with LDRD instructions, which causes the acceses
to fail when accessing the memory-mapped SPI flash.

To fix this, increase the MMIO region valid access size to allow for
64-bit accesses.

Signed-off-by: Joe Komlodi <komlodi@google.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250422002747.2593465-1-komlodi@google.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>