iio:ak8975: add mounting matrix support

Expose a rotation matrix to indicate userspace the chip orientation with
respect to the overall hardware system.
Matrix is retrieved from "in_mount_matrix". It is declared into ak8975 DTS
entry as a "mount-matrix" property.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

include/linux/iio/magnetometer/ak8975.h

@@ -0,0 +1,16 @@
+#ifndef __IIO_MAGNETOMETER_AK8975_H__
+#define __IIO_MAGNETOMETER_AK8975_H__
+
+#include <linux/iio/iio.h>
+
+/**
+ * struct ak8975_platform_data - AK8975 magnetometer driver platform data
+ * @eoc_gpio:    data ready event gpio
+ * @orientation: mounting matrix relative to main hardware
+ */
+struct ak8975_platform_data {
+	int                     eoc_gpio;
+	struct iio_mount_matrix orientation;
+};
+
+#endif