staging:iio: ak8975: add of_match table for device-tree probing

Just like isl29018; trivial addition. Using both asahi-kasei,ak8975 and
the non-prefixed version (I couldn't figure out if Asahi Kasei had a
stock symbol to use, I only found numerical indexes for their stock info).

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>