arm64: dts: qcom: ipq6018: add pcie max-link-speed

Add the generic 'max-link-speed' property to describe the IPQ6018 PCIe
link generation limit. This allows the generic dwc code to configure the
link speed correctly.

Signed-off-by: Baruch Siach <baruch.siach@siklu.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/fcf41277cf8529437374a5c10b2b1fcad30cd7c2.1640587131.git.baruch@tkos.co.il