iio: spmi-vadc: Add option to specify scale-fn-type in devicetree

There are chances that some signals would share the same ADC
channel but have different scale functions. Add an option to
specify scale-fn-type in devicetree to handle such kind of
requirements. Also modify the channel info mask to include
IIO_CHAN_INFO_PROCESSED bit if the scale-fn-type is specified
in the device tree.

Change-Id: Ib9a7c73a2cdab5e4288b3a83c2437d7e22c34636
Signed-off-by: Shilpa Suresh <sbsure@codeaurora.org>