staging: iio: cdc: ad7746: use dt for capacitive channel setup.

staging: iio: cdc: ad7746: use dt for capacitive channel setup.

Ditch platform_data fields in favor of device tree properties for
configuring EXCA and EXCB output pins and setting the capacitive channel
excitation level.

As this covers all current use cases for the platform_data struct, remove
ad7746.h header file since it's no longer needed.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Link: https://lore.kernel.org/r/3e7f2a0a8960cece185f518ff2b7ceb87891edcd.1619841953.git.lucas.p.stankus@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>