staging: ti-soc-thermal: defer probe if cpufreq is not ready

When builtin compiled, there is a chance for this driver
be probed before cpufreq driver is up and running. In this
case, the cpucooling device can be wrong initialized.

Thus, this patch makes sure this driver is probed only
when cpufreq driver is ready. Whenever there is no
cpufreq driver registered, the probe will return -EPROBE_DEFER.

Tested-by: J Keerthy <j-keerthy@ti.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>