arm64: dts: qcom: x1e80100: add bus topology for PCIe domain 3

arm64: dts: qcom: x1e80100: add bus topology for PCIe domain 3

Add pcie3_port node to represent the PCIe bridge of PCIe3 so that PCI slot
voltage rails can be described under this node in the board's dts.

Signed-off-by: Qiang Yu <qiang.yu@oss.qualcomm.com>
Signed-off-by: Wenbin Yao <quic_wenbyao@quicinc.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250722091151.1423332-3-quic_wenbyao@quicinc.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>