cpufreq: Add QCM6490 to cpufreq-dt-platdev blocklist

The Qualcomm QCM6490 platform uses the qcom-cpufreq-hw driver, so add it
to the cpufreq-dt-platdev driver's blocklist.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>