PM / OPP: OF: Use pr_debug() instead of pr_err() while adding OPP table

Some platforms add the OPPs dynamically from platform specific drivers
instead of getting them statically from DT. The cpufreq-dt driver
already ignores the return value of dev_pm_opp_of_cpumask_add_table() to
not error out for such cases, but we still end up printing error message
from that routine. That's not nice.

Convert the print message to use pr_debug() instead.

Reported-by: Mason <slash.tmp@free.fr>
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>