thermal: exynos: Fix first temperature read after registering sensor

Thermal core could not read the temperature after registering the
thermal sensor with thermal_zone_of_sensor_register() because the driver
was not yet initialized.

The call trace looked like:
    exynos_tmu_probe()
        thermal_zone_of_sensor_register()
            of_thermal_set_mode()
                thermal_zone_device_update()
                    exynos_get_temp()
                        if (!data->tmu_read) return -EINVAL;
        exynos_map_dt_data()
            data->tmu_read = ...

This produced an error in dmesg:
thermal thermal_zone0: failed to read out thermal zone (-22)

Register the thermal_zone_device later, after parsing Device Tree and
enabling necessary clocks, but before calling exynos_tmu_initialize()
which uses the registered thermal_zone_device.

Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Fixes: 3b6a1a80 ("thermal: samsung: core: Exynos TMU rework to use device tree for configuration")
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>