aspeed: fix hardcode boot address 0

aspeed: fix hardcode boot address 0

In the previous design of ASPEED SOCs QEMU model, it set the boot
address at "0" which was the hardcode setting for ast10x0, ast2600,
ast2500 and ast2400.

According to the design of ast2700, it has a bootmcu(riscv-32) which
is used for executing SPL and initialize DRAM and copy u-boot image
from SPI/Flash to DRAM at address 0x400000000 at SPL boot stage.
Then, CPUs(cortex-a35) execute u-boot, kernel and rofs.

Currently, qemu not support emulate two CPU architectures
at the same machine. Therefore, qemu will only support
to emulate CPU(cortex-a35) side for ast2700 and the boot
address is "0x4 00000000".

Fixed hardcode boot address "0" for future models using
a different mapping address.

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>