thermal: ti-soc-thermal: freeze FSM while computing trend

In order to read the history buffer, it is required to
freeze BG FSM. This patch adds the missing piece of code
to freeze the FSM and also a contention area to avoid
other parts of the code to access the DTEMPs.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>