dt-bindings: PCI: dwc: Add max-link-speed common property

In accordance with [1] DW PCIe controllers support up to Gen5 link speed.
Let's add the max-link-speed property upper bound to 5 then. The DT
bindings of the particular devices are expected to setup more strict
constraint on that parameter.

[1] Synopsys DesignWare Cores PCI Express Controller Databook, Version
5.40a, March 2019, p. 27

Link: https://lore.kernel.org/r/20221113191301.5526-7-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>