Convert TEGRA20_ defines to either TEGRA_ or NV_PA_ where appropriate.
Convert tegra20_ source file and function names to tegra_, also.

Upcoming Tegra30 port will use common code/defines/names where possible.
Signed-off-by: NTom Warren <twarren@nvidia.com>
Acked-by: NStephen Warren <swarren@nvidia.com>