dt-bindings: iio: magnetometer: ak8975: reword gpios, add interrupts, fix style

Reword gpios documentation, add interrupt documentation and fix styles.
Update example to use interrupts instead of gpios.

Signed-off-by: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>