aspeed: Add a boot_rom overlap region in the SoC spi_boot container

aspeed: Add a boot_rom overlap region in the SoC spi_boot container

To avoid the SPI transactions fetching instructions from the FMC CE0
flash device and speed up boot, a ROM can be created if a drive is
available.

Reverse the logic to allow a machine to boot without a drive, using a
block device instead :

    -blockdev node-name=fmc0,driver=file,filename=/path/to/flash.img \
    -device mx66u51235f,bus=ssi.0,drive=fmc0

Signed-off-by: Cédric Le Goater <clg@kaod.org>