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 the SMB1398 TEMP and SMB1390
TEMP use the same ADC channel but have a different voltage to
temperature calculation. Add an option to specify scale-fun-type in
devicetree to handle such kind of requirement.

Change-Id: I04ed618829f460164643c63943993ff2c522852b
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>