Staging: iio: oaktrail AK8975 support via IIO

Actually this is generic 'I have no GPIO' support for the AK8975, instead
we have to go bus polling.

Huang Liang produced an initial patch which worked by removing the support
for GPIO pins. This patch instead makes GPIO support optional and Huang then
fixed some bugs in it.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>