DELTA_DFBM_NQ620 default configuration (#3298)

* Change default SRC setting and add mbed_sdk_init() for DELTA_DFBM_NQ620

Change SRC setting to RC as default to match with hardware config.
mbed_sdk_init() is added for internal debug purpose (experimental)

* remove the redundant #define

Those #define never used.

targets/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52832/TARGET_DELTA_DFBM_NQ620/mbed_overrides.c

@@ -0,0 +1,21 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2006-2016 ARM Limited
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+void mbed_sdk_init()
+{
+	printf("", __TIME__, __DATE__);	
+	
+}

targets/targets.json

@@ -2490,6 +2490,7 @@
         "macros_add": ["BOARD_PCA10040", "NRF52_PAN_12", "NRF52_PAN_15", "NRF52_PAN_58", "NRF52_PAN_55", "NRF52_PAN_54", "NRF52_PAN_31", "NRF52_PAN_30", "NRF52_PAN_51", "NRF52_PAN_36", "NRF52_PAN_53", "S132", "CONFIG_GPIO_AS_PINRESET", "BLE_STACK_SUPPORT_REQD", "SWI_DISABLE0", "NRF52_PAN_20", "NRF52_PAN_64", "NRF52_PAN_62", "NRF52_PAN_63"],
         "device_has": ["ANALOGIN", "ERROR_PATTERN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "LOWPOWERTIMER", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SLEEP", "SPI", "SPI_ASYNCH", "SPISLAVE"],
         "release_versions": ["2", "5"],
+        "overrides": {"lf_clock_src": "NRF_LF_SRC_RC"}, 
         "device_name": "nRF52832_xxAA"
     },
     "BLUEPILL_F103C8": {