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_softdevice.go

@@ -0,0 +1,40 @@
+// +build nrf,softdevice
+
+package runtime
+
+import (
+	"device/arm"
+	"device/nrf"
+)
+
+//export sd_app_evt_wait
+func sd_app_evt_wait()
+
+func waitForEvents() {
+	// Call into the SoftDevice to sleep. This is necessary here because a
+	// normal wfe will not put the chip in low power mode (it still consumes
+	// 500µA-1mA). It is really needed to call sd_app_evt_wait for low power
+	// consumption.
+
+	// First check whether the SoftDevice is enabled. Unfortunately,
+	// sd_app_evt_wait cannot be called when the SoftDevice is not enabled.
+	var enabled uint8
+	arm.SVCall1(0x12, &enabled) // sd_softdevice_is_enabled
+
+	if enabled != 0 {
+		// Now pick the appropriate SVCall number. Hopefully they won't change
+		// in the future with a different SoftDevice version.
+		if nrf.DEVICE == "nrf51" {
+			// sd_app_evt_wait: SOC_SVC_BASE_NOT_AVAILABLE + 29
+			arm.SVCall0(0x2B + 29)
+		} else if nrf.DEVICE == "nrf52" || nrf.DEVICE == "nrf52840" {
+			// sd_app_evt_wait: SOC_SVC_BASE_NOT_AVAILABLE + 21
+			arm.SVCall0(0x2C + 21)
+		} else {
+			sd_app_evt_wait()
+		}
+	} else {
+		// SoftDevice is disabled so we can sleep normally.
+		arm.Asm("wfe")
+	}
+}