memory: tegra: Do not try to probe SMMU on Tegra20

Tegra20 doesn't have SMMU. Move out checking of the SMMU presence from
the SMMU driver into the Memory Controller driver. This change makes code
consistent in regards to how GART/SMMU presence checking is performed.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>