ARM: tegra: convert to multi-platform

This allows Tegra be included in a kernel build that supports multiple
SoCs at once, which is useful for distro kernels.

This change:
* Moves Tegra's Kconfig into its own directory, as seems typical for
  multi-platform conversions.
* Stops selecting some ARM errata that are incompatible with multi-
  platform. This requires that you use a bootloader that enables the
  workaround!
* Deletes some headers and Makefile.boot that aren't needed now that we
  support multi-platform.

Signed-off-by: Stephen Warren <swarren@nvidia.com>