Tegra20: Move some code files to common directories for upcoming Tegra30 patches.

Move files that are going to be common between T20 and T30 into 'tegra-common'
subdirs in AVP (arm720t), CPU (armv7), and shared (arch/arm/cpu/.) areas. Any
files that are left behind in '/tegra20' will be copied to '/tegra30' subdirs
and modified for that SoC. The 'common' files should need only minor changes.

Include files (arch/arm/include/asm/arch-tegra/tegra20) will be done in a
follow-on patch.

Builds fine w/MAKEALL -s tegra20. Checkpatch.pl is clean.

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