drivers: thermal: tsens: Add new operation to check if a sensor is enabled

is_sensor_enabled() checks if the sensors are enabled on this platform.
It is possible that the SoC might choose not to enable all the sensors
that the IP block is capable of supporting.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>