nrf: call sd_app_evt_wait when the SoftDevice is enabled

This reduces current consumption from 500-1000µA to very low (<10µA)
current consumption. This change is important for battery powered
devices, especially devices that may be running for long periods of
time.

src/runtime/runtime_atsamd21.go

@@ -345,3 +345,7 @@ func initADCClock() {
 		sam.GCLK_CLKCTRL_CLKEN)
 	waitForSync()
 }
+
+func waitForEvents() {
+	arm.Asm("wfe")
+}