MAINTAINERS: Add myself as Tegra PCIe maintainer

I'll be taking on maintainership of the Tegra PCIe driver since it's now
moved out of arch/arm/mach-tegra.

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