Add support for the TI / Burr-Brown INA209 voltage / current / power
monitor.

Cc: Paul Hays <haysp@magma.net>
Cc: Ira W. Snyder <iws@ovro.caltech.edu>
Tested-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>