arm64: tegra: Make DT model property consistent

Jetson Nano, Jetson TX1 and Jetson TX2 all are named "Developer Kit" and
Jetson AGX Xavier is the odd one out. It's officially also called the
"Developer Kit", not "Development Kit", so make it consistent with the
rest.

Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>