mainboard/emulation/qemu-aarch64: Add new board for ARMv8

This CL adds a new board, QEMU/AArch64, for ARMv8. The machine supported
is virt which is a QEMU 2.8 ARM virtual machine. The default CPU of
qemu-system-aarch64 is Cortex-a15, so you need to specify a 64-bit cpu
via a flag.

To execute:
$ qemu-system-aarch64 -M virt,secure=on,virtualization=on \
  -cpu cortex-a53 -bios build/coreboot.rom -m 8192M -nographic

Change-Id: Id7c0831b1ecf08785b4ec8139d809bad9b3e1eec
Signed-off-by: Asami Doi <d0iasm.pub@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33387
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

Documentation/mainboard/index.md

@@ -24,6 +24,7 @@ The boards in this section are not real mainboards, but emulators.
 
 - [Spike RISC-V emulator](emulation/spike-riscv.md)
 - [Qemu RISC-V emulator](emulation/qemu-riscv.md)
+- [Qemu AArch64 emulator](emulation/qemu-aarch64.md)
 
 ## Intel