default-configs: Add config for aarch64-softmmu

Add a config for aarch64-softmmu; this enables building of this target.
The resulting executable doesn't know about any 64 bit CPUs, but all
the 32 bit CPUs and board models work.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1385645602-18662-8-git-send-email-peter.maydell@linaro.org
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>