tracing, thermal: Hide devfreq trace events when not in use

tracing, thermal: Hide devfreq trace events when not in use

As trace events when defined create data structures and functions to
process them, defining trace events when not using them is a waste of
memory.

The trace events thermal_power_devfreq_get_power and
thermal_power_devfreq_limit are only used when CONFIG_DEVFREQ_THERMAL
is set. Make those events only defined when that is set as well.

Link: http://lkml.kernel.org/r/20171013102150.0050cb74@gandalf.local.home
Acked-by: Javi Merino <javi.merino@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>