Until now all Tegra chips except Tegra20 had a fallback if SKU is not
known. This caused issues previously when certain SKU wasn't known. Add a
fallback for Tegra20 aligning it with other Tegra SoC generations.
Signed-off-by: Ion Agorria <ion@agorria.com> Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>