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/machine/board_arduino.go

@@ -2,7 +2,10 @@
 
 package machine
 
-const CPU_FREQUENCY = 16000000
+// Return the current CPU frequency in hertz.
+func CPUFrequency() uint32 {
+	return 16000000
+}
 
 // LED on the Arduino
 const LED Pin = 13