MAINTAINERS: add Alexandre Courbot for Tegra

I'd like to propose Alexandre Courbot as an additional Tegra maintainer.
He's been working on a variety of Tegra-related code for a while, and
is now officially tasked with working on upstream support.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
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v2:
* Use Alexandre's full name.
* Use a non-NVIDIA email address to avoid Exchange Server patch corruption.
MAINTAINERS: condense some Tegra related entries

There's little point having specific entries in MAINTAINERS for Tegra
drivers that are already covered by the top-level Tegra architecture
support entry, and maintained by people listed there. Remove the
duplicates.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
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v2: No change