iio: adc: ad7291: convert to device tree

There are no in-tree users of the platform data for this driver, so
remove it and convert the driver to use device tree instead.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Link: https://lore.kernel.org/r/20200922144422.542669-1-michael.auchter@ni.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>