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_atsamd51.go

@@ -14,7 +14,9 @@ import (
 	"unsafe"
 )
 
-const CPU_FREQUENCY = 120000000
+func CPUFrequency() uint32 {
+	return 120000000
+}
 
 type PinMode uint8