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/arch_cortexm.go

@@ -102,7 +102,3 @@ func procPin() {
 func procUnpin() {
 	arm.EnableInterrupts(procPinnedMask)
 }
-
-func waitForEvents() {
-	arm.Asm("wfe")
-}