iio: support writing processed values to IIO channels

Add a new function to the IIO API which allows consumers to write
processed values to IIO channels.

This is particularly useful for supporting hardware which has
configurable ADC thresholds. A consumer would be able to change an ADC
threshold by providing a processed value instead of calculating a raw
value.

Change-Id: I7d3b22beddb6fd1fda0cc0aefbcb4cf5cb58bf82
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>