Updated Sleep

using WFE for nordic sleep
Removed systemoff call from deepleep since it does not retain program
state.

libraries/mbed/targets/hal/TARGET_NORDIC/TARGET_NRF51822/sleep.c

@@ -21,10 +21,10 @@ void sleep(void) {
     // ensure debug is disconnected if semihost is enabled....
     NRF_POWER->TASKS_LOWPWR=1;
     // wait for interrupt
-    __WFI();
+    __WFE();
 }
 
 void deepsleep(void) {
     sleep();
-    NRF_POWER->SYSTEMOFF=1;
+ //   NRF_POWER->SYSTEMOFF=1;
 }