cpu/nrf52: add CPU specific sleep function

This function is supposed to be used for putting the CPU into
sleep mode for short amounts of time (e.g. in typical polling
loops used in periph drivers).

cpu/nrf52/cpu.c

@@ -78,6 +78,9 @@ void cpu_init(void)
     NVIC_SetPriority(SWI0_EGU0_IRQn, 6);
 #endif
 
+    /* enable wake up on events for __WFE CPU sleep */
+    SCB->SCR |= SCB_SCR_SEVONPEND_Msk;
+
     /* trigger static peripheral initialization */
     periph_init();
 }

cpu/nrf52/include/cpu_conf.h

@@ -75,6 +75,16 @@ extern "C" {
 #endif /* SOFTDEVICE_PRESENT */
 /** @} */
 
+/**
+ * @brief   Put the CPU in the low-power 'wait for event' state
+ */
+static inline void nrf52_sleep(void)
+{
+    __SEV();
+    __WFE();
+    __asm("nop");
+}
+
 #ifdef __cplusplus
 }
 #endif