aspeed: Use the boot_rom region of the fby35 machine

aspeed: Use the boot_rom region of the fby35 machine

This change completes commits 5aa281d757 ("aspeed: Introduce a
spi_boot region under the SoC") and 8b744a6a47 ("aspeed: Add a
boot_rom overlap region in the SoC spi_boot container") which
introduced a spi_boot container at the SoC level to map the boot rom
region as an overlap.

It also fixes a Coverity report (CID 1508061) for a memory leak
warning when the QEMU process exits by using an bmc_boot_rom
MemoryRegion available at the machine level.

Cc: Peter Delevoryas <peter@pjd.dev>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>