ARM: shmobile: kzm9g-reference dts: Add ak8975 magnetometer node

Add a DT node for the AK8975 magnetometer sensor, which is connected to
i2c0.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Specify the device interrupt to avoid polling for end of conversion.

While at it rename the DT node to compass@c to describe the device's
function instead of its model.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>