iio: adc: sun4i-gpadc-iio: register in the thermal after registering in pm

This driver has a get_temp function used by the thermal framework that
uses pm functions.

However, the driver isn't registered in pm before it is registered in
thermal framework, resulting in the pm_resume not being called and thus
the IP not enabled.

When the IP is disabled, the raw temp value is always 0. With the
devices currently supported, it isn't a problem since with their
respective formula, they return a really cold SoC temperature which
isn't a problem for the thermal framework. But for future SoC that have
a different formula, it could return a critically hot temperature,
forcing the thermal framework to shutdown the board.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>