soc/tegra: Provide per-SoC Kconfig symbols

Move per-SoC generation Kconfig symbols to drivers/soc/tegra/Kconfig to
gather them all in a single place. This directory is a natural location
for these options since it already contains the drivers that are shared
across 32-bit and 64-bit ARM architectures.

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