staging: iio: ak8975: add platform data.

As some of the platform not support irq_to_gpio, we pass gpio port
by platform data.

Signed-off-by: Tony SIM <chinyeow.sim.xt@renesas.com>
Signed-off-by: Andrew Chew <achew@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>