Merge pull request #7073 from maciejbocianski/sleep_enable_nrf52

NRF52_DK: SLEEP enable

Author: Cruz Monrreal

targets/targets.json

@@ -3681,7 +3681,8 @@
             "SPI_ASYNCH",
             "STCLK_OFF_DURING_SLEEP",
             "TRNG",
-            "USTICKER"
+            "USTICKER",
+            "SLEEP"
         ],
         "extra_labels": [
             "NORDIC",