cpufreq: dt: Reuse dev_pm_opp_get_max_transition_latency()

OPP layer has all the information now to calculate transition latency
(clock_latency + voltage_latency). Lets reuse the OPP layer helper
dev_pm_opp_get_max_transition_latency() instead of open coding the same
in cpufreq-dt driver.

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>