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>