iio: magnetometer: ak8975: Get rid of platform data

Since IIO framework supports device property API and driver has been moved
already to the use of GPIO descriptors the logical continuation is to
get rid of platform data completely. We are on the safe side here since
there are no users of it in the kernel.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>