cpufreq: ARM big LITTLE: 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/arm_big_little_dt.txt

@@ -12,7 +12,7 @@ must be present contiguously. Generic DT driver will check only node 'x' for
 cpu:x.
 
 Required properties:
-- operating-points: Refer to Documentation/devicetree/bindings/power/opp.txt
+- operating-points: Refer to Documentation/devicetree/bindings/opp/opp.txt
   for details
 
 Optional properties: