tests/kernel/mp: Must enable KERNEL_COHERENCE explicitly
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 <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr