cpufreq: dt: Use dev_pm_opp_set_rate() to switch frequency

OPP core supports frequency/voltage changes based on the target
frequency now, use that 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>
(cherry picked from commit 78c3ba5df96c875b1668e1cd3ee0a69e62454f32)
Signed-off-by: Alex Shi <alex.shi@linaro.org>