- Use __BIT() macro to define I2C_F_* bits.
- Define an I2C_F_SPEED bitfield that can be used to specify an
  I2C bus speed:

  -> Standard Mode (100Kb/s)
  -> Fast Mode (400Kb/s)
  -> Fast Mode Plus (1Mb/s)
  -> High Speed (3.4Mb/s)

The speed values are treated as advisory; if a controller does not support
the requested speed mode, fallback to a lower one is allowed.

(Currently, all controllers simply use Standard Mode.)

(thorpej)