MAINTAINERS: tegra: Replace Stephen with Jon

For a while now, Stephen's been focussing on other areas of the open
source community and within NVIDIA. Jon Hunter's been doing a great job
keeping things working. Replace Stephen's entry with Jon's.

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