Currently the imx_sc driver is reimplementing part of the thermal zone
parsing from the thermal OF tree code to get the sensor id associated
with a thermal zone sensor.

The driver platform specific code should know what sensor is present
and not rely on the thermal zone description to do a discovery. Well
that is arguable but all the other drivers have a per platform data
telling what sensor id to use.

The imx_sc thermal driver is the only one using a different
approach. Not invalid but forcing to keep a specific function
'thermal_zone_of_get_sensor_id()' to get the sensor id for a specific
thermal zone as the self-explanatory function tells and having device
tree code inside the driver.

The thermal OF code had a rework and remains now self-encapsulated
with a register/unregister functions and their 'devm' variants, except
for the function mentioned above.

After investigating, it appears the imx_sc sensor is defined in
arch/arm64/boot/dts/freescale/imx8qxp.dtsi:

which defines the cpu-thermal zone with the id: IMX_SC_R_SYSTEM

This dtsi is included by:
 - imx8qxp-ai_ml.dts
 - imx8qxp-colibri.dtsi
 - imx8qxp-mek.dts

The two first ones do not define more thermal zones
The third one adds the pmic-thermal0 zone with id: IMX_SC_R_PMIC_0

The thermal OF code returns -ENODEV if the thermal zone registration
with a specific id fails because the description is not available in
the DT for such a sensor id. In this case we continue with the other
ids without bailing out with an error.

So we can build for the 'fsl,imx-sc-thermal' a compatible data, an
array of sensor ids containing IMX_SC_R_SYSTEM and IMX_SC_R_PMIC_0.

The latter won't be found but that will not result in an error but a
normal case where we continue the initialization with other ids.

Just to clarify, it is what the thermal framework does and what the
other drivers are expecting: when a registration fails with -ENODEV
this is not an error but a case where the description is not found in
the device tree, that be can the entire thermal zones description or a
specific thermal zone with an unknown id.

There is one small functional change but without impact. When there is
no 'thermal-zones' description the probe function was returning
'-ENODEV', now it returns zero. When a thermal zone fails to register
with an error different from '-ENODEV', the error is detected and
returned.

Change the code accordingly and remove the OF code from the driver.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20220818082316.2717095-1-daniel.lezcano@linaro.org