aspeed: Introduce a spi_boot region under the SoC

aspeed: Introduce a spi_boot region under the SoC

The default boot address of the Aspeed SoCs is 0x0. For this reason,
the FMC flash device contents are remapped by HW on the first 256MB of
the address space. In QEMU, this is currently done in the machine init
with the setup of a region alias.

Move this code to the SoC and introduce an extra container to prepare
ground for the boot ROM region which will overlap the FMC flash
remapping.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>