soc/tegra: Implement runtime check for Tegra SoCs

soc/tegra: Implement runtime check for Tegra SoCs

Subsequent patches will move some of the initialization code from SoC
setup code to regular initcalls. To prevent breakage on other SoCs in
multi-platform builds, these initcalls need to check that they indeed
run on Tegra.

Signed-off-by: Thierry Reding <treding@nvidia.com>