opp: Print OPP level in debug message of _opp_add_static_v2()

Print OPP level in debug message of _opp_add_static_v2(). This helps to
chase GENPD bugs.

Tested-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Nicolas Chauvet <kwizart@gmail.com>
Tested-by: Matt Merhar <mattmerhar@protonmail.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>