machine: rename CPU_FREQUENCY -> CPUFrequency()

These all-caps constants aren't in the Go style, so rename it to
CPUFrequency (which is more aligned with Go style). Additionally, make
it a function so that it is possible to add support for changing the
frequency in the future.

Tested by running `make smoketest`. None of the outputs did change.

src/examples/systick/systick.go

@@ -9,7 +9,7 @@ func main() {
 	machine.LED.Configure(machine.PinConfig{Mode: machine.PinOutput})
 
 	// timer fires 10 times per second
-	arm.SetupSystemTimer(machine.CPU_FREQUENCY / 10)
+	arm.SetupSystemTimer(machine.CPUFrequency() / 10)
 
 	for {
 	}