tests: kernel/mp: move CONFIG_KERNEL_COHERENCE to Kconfig

This moves CONFIG_KERNEL_COHERENCE from prj.conf to Kconfig.
This gets rid of the cmake warning where CONFIG_KERNEL_COHERENCE
is assigned the value 'y' but gets the value ''. This is simply
done to avoid confusion when running the test manually.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>