Enable us/lp ticker support for NRF51_DK board

Author: mprse

targets/targets.json

@@ -3441,7 +3441,8 @@
             "TARGET_MCU_NRF51822",
             "CMSIS_VECTAB_VIRTUAL",
             "CMSIS_VECTAB_VIRTUAL_HEADER_FILE=\"cmsis_nvic.h\"",
-            "NO_SYSTICK"
+            "NO_SYSTICK",
+            "MBED_TICKLESS"
         ],
         "MERGE_BOOTLOADER": false,
         "extra_labels": ["NORDIC", "MCU_NRF51", "MCU_NRF51822_UNIFIED", "NRF5", "SDK11"],
@@ -3486,8 +3487,8 @@
     "NRF51_DK": {
         "supported_form_factors": ["ARDUINO"],
         "inherits": ["MCU_NRF51_32K_UNIFIED"],
-        "device_has": ["ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPI_ASYNCH", "SPISLAVE"],
-        "release_versions": ["2"],
+        "device_has": ["USTICKER", "LPTICKER", "ANALOGIN", "I2C", "I2C_ASYNCH", "INTERRUPTIN", "PORTIN", "PORTINOUT", "PORTOUT", "PWMOUT", "RTC", "SERIAL", "SERIAL_ASYNCH", "SERIAL_FC", "SLEEP", "SPI", "SPI_ASYNCH", "SPISLAVE"],
+        "release_versions": ["2", "5"],
         "device_name": "nRF51822_xxAA"
     },
     "NRF51_DONGLE": {