device/arm: add system timer registers (#654)

* device/arm: add system timer registers
Add SYST registers and bit definitions to device/arm.
Add a setup function.
Add an example that uses it to blink an LED.

src/examples/systick/systick.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"device/arm"
+	"machine"
+)
+
+func main() {
+	machine.LED.Configure(machine.PinConfig{Mode: machine.PinOutput})
+
+	// timer fires 10 times per second
+	arm.SetupSystemTimer(machine.CPU_FREQUENCY / 10)
+
+	for {
+	}
+}
+
+var led_state bool
+
+//go:export SysTick_Handler
+func timer_isr() {
+	if led_state {
+		machine.LED.Low()
+	} else {
+		machine.LED.High()
+	}
+	led_state = !led_state
+}