Add MBED_SPM label to K64F

Ensure that K64F will continue to receive PSA defines

Author: Oren Cohen

targets/targets.json

@@ -1452,7 +1452,8 @@
             "KPSDK_CODE",
             "MCU_K64F",
             "Freescale_EMAC",
-            "PSA"
+            "PSA",
+            "MBED_SPM"
         ],
         "is_disk_virtual": true,
         "macros": ["CPU_MK64FN1M0VMD12", "FSL_RTOS_MBED", "MBEDTLS_PSA_CRYPTO_C"],