MAINTAINERS: Update tegra maintainer information

A couple of changes to the Tegra maintainership setup:

I'm very glad to bring on Stephen Warren on board as a maintainer. The
work he has done so far is excellent, and the fact that he works for
Nvidia means he has long-term interest in the platform.

Erik Gilling did an astounding amount of work on getting things up and
running but has been a silent partner on the maintainership side for a
while, and is stepping down. Thanks for your contributions so far, Erik.

Finally, update the git URL since I'll take over running the main repo
for a while.

Overall maintainership model isn't changing much at this time: We'll all
three review patches as appropriate, and one of us will collect the main
repo (me at this time).

Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Erik Gilling <konkers@android.com>
Acked-by: Colin Cross <ccross@android.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>