OPP: Improve _find_table_of_opp_np()

Make _find_table_of_opp_np() more efficient by using of_get_parent() to
find the parent OPP table node.

Tested-by: Rajendra Nayak <rnayak@codeaurora.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>