tegra20: move tegra20 SoC code to arch/arm/cpu/tegra20-common

In preparation for splitting out the armv4t code from tegra20, move
the tegra20 SoC code to arch/arm/cpu/tegra20-common.  This code will
be compiled armv4t for the arm7tdmi and armv7 for the cortex A9.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>