arm64: dts: qcom: sc7180: Add labels for cpuN-thermal nodes

Add labels to the cpuN-thermal nodes to allow board files to use
a phandle instead replicating the node hierarchy when adjusting
certain properties.

Due to the 'sustainable-power' property CPU thermal zones are
more likely to need property updates than other SC7180 zones,
hence only labels for CPU zones are added for now.

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20210108141648.1.Ia8019b8b303ca31a06752ed6ceb5c3ac50bd1d48@changeid
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>