OPP: Don't drop opp->np reference while it is still in use

The struct dev_pm_opp contains a reference of the DT node, opp->np,
throughout its lifetime. We should increase the refcount for the same
from _opp_add_static_v2(), and drop it while removing the OPP finally.

Signed-off-by: Liang He <windhl@126.com>
[ Viresh: Updated subject / commit log, create _of_clear_opp() and drop
	  reference from it]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>