tegra2: Enable data cache

This enables the data cache on Tegra2 boards.

As discussed on the list, this is better off in the Tegra2 cpu code than in a
particular vendor directory. We should be safe turning on the cache for all
Tegra2 boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>