aspeed/smc: Use a container for the flash mmio address space

aspeed/smc: Use a container for the flash mmio address space

Because AddressSpaces must not be sysbus-mapped, commit e9c568dbc225
("hw/arm/aspeed: Do not sysbus-map mmio flash region directly, use
alias") introduced an alias for the flash mmio region.

Using a container is cleaner.

Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-Id: <20211018132609.160008-5-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>