iio: adc: spmi-vadc: Add adc support for SPF targets

Update the channel type and scaling function for the VADC
channels that are used by legacy SPF targets.

Change-Id: I6637388ace022c5726a8f00fb1f8c0eb98c39f5c
Signed-off-by: Asha Magadi Venkateshamurthy <amagad@codeaurora.org>
Signed-off-by: Archana Sriram <apsrir@codeaurora.org>