dt-bindings: iio: Add scale function for PM2250 S3 die temp

Add scale function macro for selecting correct scale function for S3
die temp ADC channel on PM2250 V2, as the channel is mapped to the
default voltage scaling function otherwise.

Change-Id: I6c4905100a19adcc0cf9d6beb9cb3471cc4d232b
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>