No I2C support for stm32g4

[omarandlorraine]

Judging by the docs the stm32g4 has no i2c support in libopencm3. But I need it. Time permitting I'll put it in in the next few weeks.

It would be my first time of contributing to libopencm3. So I don't know what I'm getting myself into. So I need to check first if there has been any work towards this from anyone else, or if anyone knows where I should look first

[qosch]

This is covered by #1250 and #1416