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_nxpmk66f18.go

@@ -265,3 +265,7 @@ func abort() {
 		machine.PollUART(&machine.UART2)
 	}
 }
+
+func waitForEvents() {
+	arm.Asm("wfe")
+}