Thermal: initialize thermal zone device correctly

After thermal zone device registered, as we have not read any
temperature before, thus tz->temperature should not be 0,
which actually means 0C, and thermal trend is not available.
In this case, we need specially handling for the first
thermal_zone_device_update().

Both thermal core framework and step_wise governor is
enhanced to handle this. And since the step_wise governor
is the only one that uses trends, so it's the only thermal
governor that needs to be updated.

CC: <[email protected]> #3.18+
Tested-by: Manuel Krause <[email protected]>
Tested-by: szegad <[email protected]>
Tested-by: prash <[email protected]>
Tested-by: amish <[email protected]>
Tested-by: Matthias <[email protected]>
Reviewed-by: Javi Merino <[email protected]>
Signed-off-by: Zhang Rui <[email protected]>
Signed-off-by: Chen Yu <[email protected]>

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