MAINTAINERS: Add Mikko as backup maintainer for Tegra DRM

Mikko has been involved as the primary author of the host1x driver and
has volunteered to help out with maintenance.
Signed-off-by: Thierry Reding <treding@nvidia.com>

MAINTAINERS

@@ -6916,6 +6916,7 @@ F:	drivers/phy/mediatek/phy-mtk-mipi*
 
 DRM DRIVERS FOR NVIDIA TEGRA
 M:	Thierry Reding <thierry.reding@gmail.com>
+M:	Mikko Perttunen <mperttunen@nvidia.com>
 L:	dri-devel@lists.freedesktop.org
 L:	linux-tegra@vger.kernel.org
 S:	Supported