boards: mps2-an385: this platform is supported by qemu

In Qemu 2.10 we can run this board configuration in Qemu directly. Make
this possible by setting the board configuration to allow emulation.

Until we have the latest QEMU in the SDK this can be used with a recent
QEMU like this:

$ mkdir build; cd build
$ cmake -DQEMU=/usr/bin/qemu-system-arm -DBOARD=mps2_an385 ..
$ make run

Signed-off-by: Anas Nashif <anas.nashif@intel.com>