arm64: dts: qcom: sc8180x: Add a SoC-specific compatible to cpufreq-hw

Comply with bindings guidelines and get rid of errors such as:

cpufreq@18323000: compatible: 'oneOf' conditional failed, one must be fixed:
        ['qcom,cpufreq-hw'] is too short

Fixes: 8575f197 ("arm64: dts: qcom: Introduce the SC8180x platform")
Signed-off-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>