iio: qcom,spmi-vadc: Add DT includes for qcom,scale-fn-type

Add defined values which can be used to be specified in
"qcom,scale-fn-type" property.

Change-Id: I960e0b8761256545cc2e07bb8c260b990d416bab
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>