stm32f4: i2c: primitive example of reading/writing regs

Uses the onboard CS43L22 codec, so no external parts or wiring is required.
This demo should scare you away from I2C.
stm32f4: i2c: include Logic trace capture
stm32l1: i2c: clone the f4 demo to test i2c works

Probably not a generally useful demo, requires a lot of wiring to an F4 board.