Add I2C support to the Zephyr builds

[tannewt]

You cannot instantiate zephyr_i2c.I2C() because they are fixed by the board's device tree. All busses with status = "okay" are added to board with all of their labels as names. So, I2C() may be the same as I2C1() and ARDUINO_I2C().

Part of #9903.

[dhalbert]

Thanks! Looks good to me, but did not test.

It's too bad there's so much duplicated code and documentation. If the documentation is largely the same for busio.I2C, I wonder if it should cross-reference that instead of copying it. The shared-binding code of busio.I2C vs zephyr_i2c.I2C also seems to be pretty much identical, or are there subtle differences? I don't see how to get around that except maybe if one macro-ified the the shared-binding source so it could be instantiated several different ways. But it's quite messy to do that in C.

meant to Request Changes

[dhalbert]

One possible change in doc.

[tannewt]

It's too bad there's so much duplicated code and documentation. If the documentation is largely the same for busio.I2C, I wonder if it should cross-reference that instead of copying it. The shared-binding code of busio.I2C vs zephyr_i2c.I2C also seems to be pretty much identical, or are there subtle differences?

I think this is a bit of a trade off with having a shared central doc file versus having it in shared bindings. Having it next to the shared-bindings means that it needs to be duplicated.

We could extract it out and make a fake superclass with the shared API for the docs. bitbangio has a copy of these docs too. Of course maybe we should figure out how to share the C code too so the implementation is the same too. I know bitbangio's implementation drifted from busio for a time but the docs did still match because they were duplicated.

[dhalbert]

It's fine to merge this now. We can think about how to share documentation (maybe there's a Sphinx feature?? or maybe used #include and how to share C code. C code is harder because the function names are different even if everything else is the same.