iio: ina2xx: add support for TI INA2xx Power Monitors

in SOFTWARE buffer mode, a kthread will capture the active scan_elements
into a kfifo, then compute the remaining time until the next capture tick
and do an active wait (udelay).

This will produce a stream of up to fours channels plus a 64bits
timestamps (ns).

Tested with ina226, on BeagleBoneBlack.

Datasheet: http://www.ti.com/lit/gpn/ina226



Signed-off-by: Marc Titinger <mtitinger@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>