thermal: of: Enable thermal_zoneX when sensor is correctly added

Up till now the thermal_zone mode was by default "disabled". With this
patch the default behavior was changed to "enable".

One can read the mode at:
/sys/class/thermal/thermal_zone0/mode

Tested-by: Javi Merino <javi.merino@arm.com>
Reported-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>