arm/tegra: rename board-dt.c to board-dt-tegra20.c

Tegra20 based boards will be handled by the current board-dt.c file. Tegra30
based boards will be handled by a new board-dt-tegra30.c file. Hence rename
the existing board-dt.c to board-dt-tegra20.c to reflect its use.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Colin Cross <ccross@android.com>
Signed-off-by: Olof Johansson <olof@lixom.net>