aspeed: Add an AST2700 eval board

aspeed: Add an AST2700 eval board

AST2700 CPU is ARM Cortex-A35 which is 64 bits.
Add TARGET_AARCH64 to build this machine.

According to the design of ast2700, it has a bootmcu(riscv-32) which
is used for executing SPL.
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

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>