arm64: dts: qcom: msm8996: Use node references in db820c

Instead of mimicing the structure of the platform, reference nodes by
their label in apq8096-db820c.dtsi. Add labels in msm8996.dtsi where
necessary.

Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>