test/avocado/machine_aspeed.py: Add AST2700 test case

test/avocado/machine_aspeed.py: Add AST2700 test case

Add a test case to test Aspeed OpenBMC SDK v09.01 on AST2700 board.

It loads u-boot-nodtb.bin, u-boot.dtb, tfa and optee-os
images to dram first which base address is 0x400000000.
Then, boot and launch 4 cpu cores.

```
qemu-system-aarch64 -machine ast2700-evb
    -device loader,force-raw=on,addr=0x400000000,file=workdir/u-boot-nodtb.bin \
    -device loader,force-raw=on,addr=uboot_dtb_load_addr,file=workdir/u-boot.dtb\
    -device loader,force-raw=on,addr=0x430000000,file=workdir/bl31.bin\
    -device loader,force-raw=on,addr=0x430080000,file=workdir/optee/tee-raw.bin\
    -device loader,cpu-num=0,addr=0x430000000 \
    -device loader,cpu-num=1,addr=0x430000000 \
    -device loader,cpu-num=2,addr=0x430000000 \
    -device loader,cpu-num=3,addr=0x430000000 \
    -smp 4 \
    -drive file=workdir/image-bmc,format=raw,if=mtd
```

A test image is downloaded from the ASPEED Forked OpenBMC GitHub release repository :
https://github.com/AspeedTech-BMC/openbmc/releases/

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>