dt-bindings: iio: Add scale function for QM215 batt therm

Add scale function macro for selecting correct scale function for
batt therm ADC channel on QM215, as the channel is mapped to the
default voltage scaling function otherwise.
Also correct the indexing for other scaling macros to match the
header file.

Change-Id: I6f72d5386dff61e834e01221ad3df2cd3d3bc1f8
Signed-off-by: Shilpa Suresh <sbsure@codeaurora.org>