arm64: dts: qcom: sc7280: Fix cpufreq-epss compatible

The bindings require a SoC-specific compatible to be used next to
qcom,cpufreq-epss. Add it to make dtbs_check happy.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221016090035.565350-2-luca@z3ntu.xyz