cpufreq: cpufreq-dt: adjust message related to regulators

The cpufreq-dt driver tries to get a regulator for each CPU. This
regulator is optional, but when not present, a scary message "failed
to get cpuX regulator" is displayed. To solve this, we reduce the
severity of the message from dev_warn() to dev_dbg() and we reword the
message to not be as scary.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>