memory: tegra: Support derated timings on Tegra210

Derated timings are used to ensure that the memory chips keep operating
correctly at high temperatures. This adds code to support polling of the
chip operating state when high temperatures are measured on the chip and
change the refresh mode accordingly. Under very high temperatures, the
driver will switch to the derated tables to ensure proper operation of
the memory chips.

Signed-off-by: Thierry Reding <treding@nvidia.com>