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_cortexm_qemu.go

@@ -50,3 +50,7 @@ var stdoutWrite = (*volatile.Register8)(unsafe.Pointer(uintptr(0x4000c000)))
 func putchar(c byte) {
 	stdoutWrite.Set(uint8(c))
 }
+
+func waitForEvents() {
+	arm.Asm("wfe")
+}