iio: adc: xilinx-xadc: Make use of device properties

Convert the module to be property provider agnostic and allow
it to be used on non-OF platforms.

Add mod_devicetable.h include.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20220531141118.64540-1-andriy.shevchenko@linux.intel.com


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