thermal: rockchip: Enable hwmon

By default, of-based thermal drivers do not enable hwmon.
Explicitly enable hwmon for both, the soc and gpu temperature
sensor.

Signed-off-by: Stefan Schaeckeler <schaecsn@gmx.net>
Tested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191212061702.BFE2D6E85603@corona.crabdance.com