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_nrf_bare.go

@@ -0,0 +1,9 @@
+// +build nrf,!softdevice
+
+package runtime
+
+import "device/arm"
+
+func waitForEvents() {
+	arm.Asm("wfe")
+}