PM / OPP: Add 'struct kref' to OPP table

Add kref to struct opp_table for easier accounting of the OPP table.

Note that the new routine dev_pm_opp_get_opp_table() takes the reference
from under the opp_table_lock, which guarantees that the OPP table
doesn't get freed unless dev_pm_opp_put_opp_table() is called for the
OPP table.

Two separate release mechanisms are added: locked and unlocked. In
unlocked version the routines aren't required to take/drop
opp_table_lock as the callers have already done that. This is required
to avoid breaking git bisect, otherwise we may get lockdeps between
commits. Once all the users of OPP table are updated the unlocked
version shall be removed.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>