Add a device_name to microbit entry in targets.json

Author: sarahmarshy

targets/targets.json

@@ -1616,7 +1616,8 @@
     "NRF51_MICROBIT": {
         "inherits": ["MCU_NRF51_16K_S110"],
         "macros_add": ["TARGET_NRF_LFCLK_RC"],
-        "release_versions": ["2"]
+        "release_versions": ["2"],
+        "device_name": "nRF51822_xxAA"
     },
     "NRF51_MICROBIT_BOOT": {
         "inherits": ["MCU_NRF51_16K_BOOT_S110"],