Fix errors on NCS36510 after deep sleep

Add NOPs after deep sleep to prevent unexpected behavior. It appears
that the first one or two instructions after deep sleep do not get
executed properly.

Note - This is a temporary workaround. For a more robust solution
the NCS36510 needs to investigate the root cause of this issue.

targets/TARGET_ONSEMI/TARGET_NCS36510/sleep.c

@@ -64,6 +64,8 @@ void fncs36510_deepsleep(void)
     /** Enter into deep sleep mode */
     __ISB();
     __WFI();
+    __NOP();
+    __NOP();
 
     /** Wait for the external 32MHz to be power-ed up & running
      * Re-power down the 32MHz internal osc