ARM: tegra: unify Tegra186 Makefile a bit

Many files in arch/arm/mach-tegra are compiled conditionally based on
Kconfig variables, or applicable to all platforms. We can let the main
Tegra Makefile handle compiling (or not) those files to avoid each SoC-
specific Makefile needing to duplicate entries for those files. This
leaves the SoC-specific Makefiles to compile truly SoC-specific code.

In the future, we'll hopefully add Kconfig variables for all the other
files, and refactor those files, and so reduce the need for SoC-specific
Makefiles and/or ifdefs in the Makefiles.

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