drivers: iio: enable compile for TDK sensor

This change is for:
    1. Add option in Kconfig and Makefile for TDK sensor
    2. Enable compile for TDK sensor
    3. Clean up unused variable and function in driver

Change-Id: I00e7d3e2ab7b536f22b12ae00088b97ad95f7eef
Signed-off-by: Chaojun Wang <chaojunw@codeaurora.org>