This makes it possible to assign I2C objects (machine.I2C0,
machine.I2C1, etc.) without needing to take a pointer.
This is important especially in the future when I2C may be driven using
DMA and the machine.I2C type needs to store some state.
Let's use the same default frequency everywhere, for consistency.
It could be any frequency, but 4MHz is already used for other chips and
it seems like a reasonable frequency to me (not too fast for most chips
but still reasonably fast). Oh, and 4MHz is slow enough that it can be
inspected by a Saleae Logic 4 (that sadly has been discontinued).
Ayke van Laethem
deadprogram
Yannis Huber