tests/kernel/mp: Must enable KERNEL_COHERENCE explicitly

This is test assumes that shared static/global variables are coherent
between the CPUs.  That's true on incoherent platforms only when
CONFIG_KERNEL_COHERENCE=y.  Normally that gets turned on along with
SMP, but this is using the lower level mp API directly and didn't have
that.

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