The INA219 is a combined voltage and current sensor that can measure up to 26 volts and a current that depends on an external shunt resistor.  Connection happens via i2c/smbus and the chip features a power supply rail that is independent from the measured voltage, so it can measure low voltages.

This commit adds a driver for this chip.  Right now it measures bus voltage and current, and does not use the internal calibrated current reading, nor the available power measurement.