iio:ad5064: Add support for ltc2617 and similar devices

The Linear Technology LTC2606, LTC2607, LTC2609, LTC2616, LTC2617,
LTC2619, LTC2626, LTC2627 and LTC2629 devices are very similar
to the AD5064 device.

This patch adds support for those devices.

Datasheet for LTC devices:
LTC2606, LTC2616, LTC2626: http://www.linear.com/docs/6398
LTC2607, LTC2617, LTC2627: http://www.linear.com/docs/8977
LTC2709, LTC2619, LTC2629: http://www.linear.com/docs/8477



Signed-off-by: Marc Andre <marc.andre@netline.ch>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>