cpufreq: blacklist SC7280 in cpufreq-dt-platdev

Add SC7280 to cpufreq-dt-platdev blacklist since the actual scaling is
handled by the 'qcom-cpufreq-hw' driver.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>