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_atmega.go

@@ -26,7 +26,7 @@ type I2CConfig struct {
 func (i2c *I2C) Configure(config I2CConfig) error {
 	// Default I2C bus speed is 100 kHz.
 	if config.Frequency == 0 {
-		config.Frequency = TWI_FREQ_100KHZ
+		config.Frequency = 100 * KHz
 	}
 
 	// Activate internal pullups for twi.