boards/x86/acrn_ehl_crb: Enable SMP by default

SMP is working now, make a 2-cpu configuration default for this
device.

Note that this requires changes to the default ACRN build
configuration!  In hybrid.xml, you need to specify multiple physical
CPUs for the VM to uses, e.g.:

    <vm id="0">
        ...
        <cpu_affinity>
            <pcpu_id>0</pcpu_id>
            <pcpu_id>1</pcpu_id>
        </cpu_affinity>
    </vm>

Failing to build with this change will result in the system hanging at
boot trying to start up a CPU that won't run.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>