cpufreq: blocklist MSM8998 in cpufreq-dt-platdev

Add the MSM8998 to the blocklist since the CPU scaling on this platform
is handled by a separate driver.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Tested-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>