iio: sx9500: add final devicetree support

This makes sx9500 driver usable on devicetree based platforms too.

Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Vlad Dogaru <ddvlad@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>