PM / devfreq: tegra: Add Dmitry as a maintainer

I was contributing to the NVIDIA Tegra20+ devfreq drivers recently and
want to help keep them working and evolving in the future.

Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>