arm64: dts: qcom: ipq8074: set Gen2 PCIe pcie max-link-speed

Add the generic 'max-link-speed' property to describe the Gen2 PCIe link
generation limit.
This allows the generic DWC code to configure the link speed correctly.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230113164449.906002-4-robimarko@gmail.com