dt-bindings: iio: Add bindings for sx9310 sensor

Adds device tree bindings for sx9310 sensor.

Signed-off-by: Daniel Campello <campello@chromium.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Rob Herring <robh+dt@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
[swboyd@chromium.org: Add both regulators and make them optional]
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>