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>

tests/kernel/mp/Kconfig

@@ -0,0 +1,11 @@
+# Copyright (c) 2023 Intel Corporation
+#
+# SPDX-License-Identifier: Apache-2.0
+
+# Must have this on where available, otherwise the linker will place
+# the shared variables in cached/incoherent memory.
+config KERNEL_COHERENCE
+	bool
+	default y if ARCH_HAS_COHERENCE
+
+source "Kconfig.zephyr"

tests/kernel/mp/prj.conf

@@ -1,7 +1,3 @@
 CONFIG_ZTEST=y
 CONFIG_ZTEST_NEW_API=y
 CONFIG_SMP=n
-
-# Must have this on where available, otherwise the linker will place
-# the shared variables in cached/incoherent memory.
-CONFIG_KERNEL_COHERENCE=y