ARM: tegra: remove CONFIG_MACH_TEGRA_DT

* Make ARCH_TEGRA select USE_OF; DT is the way forward.
* Build board-dt-tegra*.c when the relevant Tegra SoC support is enabled,
  rather than requiring a specific config option for this.
* The board-specific config options already build board-*-pinmux.o, and
  when booting from device tree these files are no longer needed, so we
  can remove some Makefile commands related to those files.

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