iio: core: drop of.h from iio.h

There is no reason to include OF as we only need to forward declare
'of_phandle_args'. Previously, some drivers were actually relying on
this for some headers (those were already fixed).

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220610084545.547700-20-nuno.sa@analog.com


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