Documentation/devicetree: Add PCIe max-link-speed property

Some of the host drivers have the requirement of knowing whether the EP
would never train at some link speed at all.  For instance, on some boards,
the link won't train at 5 GT/s but the host driver still sacrifice some
cycles to wait for the result of training at 5 GT/s as the host could
actually support 5 GT/s.  So we could parse this new property and make the
host drivers be aware of these cases.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>