iio: spmi-adc5: Add option to specify scale-fun-type in devicetree

There are chances that some signals would share the same ADC channel
but have different scale functions, such as SMB1398 TEMP signal would
use the same ADC channel that SMB1390 TEMP signal is using but it has
different voltage to temperature calcuation. Add an option to specify
scale-fun-type in devicetree to handle such kind of requirement.

Change-Id: I3cbf042e13a7c1b6473911154282d455231ea608
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>