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/ina226Signed-off-by: NMarc Titinger <mtitinger@baylibre.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>