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/machine_nrf52.go

@@ -7,7 +7,9 @@ import (
 	"unsafe"
 )
 
-const CPU_FREQUENCY = 64000000
+func CPUFrequency() uint32 {
+	return 64000000
+}
 
 // Get peripheral and pin number for this GPIO pin.
 func (p Pin) getPortPin() (*nrf.GPIO_Type, uint32) {