cpufreq: dt-platdev: Blacklist NVIDIA Tegra20 and Tegra30 SoCs

Both NVIDIA Tegra20 and Tegra30 SoCs should be blacklisted because CPU
OPPs use supported_hw and thus platdev isn't suitable for these SoCs.
Currently cpufreq-dt driver produces a bit annoying warning splats
during boot because valid OPPs are not found, this will be fixed once
tegra20-cpufreq driver will be update to support cpufreq-dt. The warnings
will also happen on older stable kernels using newer device-trees, thus
this patch should be backported to stable kernels as well.

Cc: <stable@vger.kernel.org>
Reported-by: Jon Hunter <jonathanh@nvidia.com>
Fixes: 4053aa65 ("ARM: tegra: cardhu-a04: Add CPU Operating Performance Points")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>