memory: tegra: Don't invoke Tegra30+ specific memory timing setup on Tegra20

This fixes irrelevant "tegra-mc 7000f000.memory-controller: no memory
timings for RAM code 0 registered" warning message during of kernels
boot-up on Tegra20.

Fixes: a8d502fd ("memory: tegra: Squash tegra20-mc into common tegra-mc driver")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>