MAINTAINERS: tegra: remove Olof/Colin, add device tree files

At Linux Plumbers conference, Olof and Colin both told me to remove them
from the MAINTAINERS file. Thank you both very much for all the help with
Tegra!

Also, include all Tegra-related device-tree files in the file list.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Colin Cross <ccross@android.com>