arch: arm: add_subdirectory shouldn't depend on a hidden kconfig.

Previously the directory core/mpu/ was getting included on a hidden
kconfig. Now this has been replaced with the Kconfig
CONFIG_ARC_CORE_MPU.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>

arch/arc/core/CMakeLists.txt

@@ -23,5 +23,5 @@ zephyr_library_sources_ifdef(CONFIG_ARC_FIRQ fast_irq.S)
 
 zephyr_library_sources_if_kconfig(irq_offload.c)
 zephyr_library_sources_ifdef(CONFIG_ATOMIC_OPERATIONS_CUSTOM atomic.c)
-add_subdirectory_ifdef(CONFIG_CPU_HAS_MPU mpu)
+add_subdirectory_ifdef(CONFIG_ARC_CORE_MPU mpu)
 zephyr_library_sources_ifdef(CONFIG_USERSPACE userspace.S)