PM / OPP: Split out part of _add_opp_table() and _remove_opp_table()

Split out parts of _add_opp_table() and _remove_opp_table() into
separate routines. This improves readability as well.

Should result in no functional changes.

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>