MAINTAINERS: Tegra updates, and driver ownership

Overhaul of MAINTAINERS for Tegra. This adds Thierry as a Tegra core
maintainer, and adds specific entries for most individual Tegra-specific
device drivers, pointing at relevant people. The tegradrm section is
updated to be Supported since Thierry is now employed to work on this.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>