qemu: enable for aarch64

Enable the target qemu for aarch64 since it works just fine.
Only tested using userland CPU emulation (no HYP) with a
qemu_aarch64_virt_defconfig image inside a qemu_aarch64_virt_defconfig
instance.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>