machine: add KHz, MHz, GHz constants, deprecate TWI_FREQ_* constants

There are two main issues with these constants:

  * They don't follow the Go naming convention.
  * They call themselves "TWI", while it makes a lot more sense to refer
    to the actual name which is I2C (or I²C).

I have not removed them but just deprecated them. Perhaps we can remove
them when we move towards version 1.0.

src/machine/machine_rp2040_clocks.go

@@ -10,11 +10,6 @@ import (
 	"unsafe"
 )
 
-const (
-	KHz = 1000
-	MHz = 1000000
-)
-
 func CPUFrequency() uint32 {
 	return 125 * MHz
 }