memory: tegra: Pre-configure debug register on Tegra20

The driver expects certain debug features to be disabled in order to
work properly. Let's disable them explicitly for consistency and to not
rely on a boot state.

Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>