cpufreq-dt: correct dead link in documentation

commit 3566c5b2 ("PM / OPP: Create a directory for opp bindings")
renamed the file:
Documentation/devicetree/bindings/power/opp.txt
to
Documentation/devicetree/bindings/opp/opp.txt
leaving a dead link in cpufreq/arm_big_little_dt.txt.

The link points now to the good file.
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt

@@ -11,7 +11,7 @@ Required properties:
 - None
 
 Optional properties:
-- operating-points: Refer to Documentation/devicetree/bindings/power/opp.txt for
+- operating-points: Refer to Documentation/devicetree/bindings/opp/opp.txt for
   details. OPPs *must* be supplied either via DT, i.e. this property, or
   populated at runtime.
 - clock-latency: Specify the possible maximum transition latency for clock,