dt-bindings: PCI: pci-ep: Extend max-link-speed to PCIe Gen5/Gen6

dt-bindings: PCI: pci-ep: Extend max-link-speed to PCIe Gen5/Gen6

Update the PCI Endpoint (EP) device tree binding documentation to
include PCIe Gen5 and Gen6 support for the `max-link-speed` property.
Similar to the Host Controller binding, the original EP binding
limited this value to 1~4 (Gen1~Gen4). With current SoCs requiring
Gen5/Gen6 support (e.g., Synopsys/Cadence IP), this change aligns
the EP binding with the kernel's PCIe 6.0 capabilities.

Signed-off-by: Hans Zhang <18255117159@163.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250529021026.475861-3-18255117159@163.com