PCI: rockchip-ep: Improve link training

The Rockchip RK3399 TRM V1.3 Part2, Section 17.5.8.1.2, step 7,
describes the endpoint mode link training process clearly and states
that:

  Insure link training completion and success by observing link_st field
  in PCIe Client BASIC_STATUS1 register change to 2'b11. If both side
  support PCIe Gen2 speed, re-train can be Initiated by asserting the
  Retrain Link field in Link Control and Status Register. The software
  should insure the BASIC_STATUS0[negotiated_speed] changes to "1", that
  indicates re-train to Gen2 successfully.

This procedure is very similar to what is done for the root-port mode
in rockchip_pcie_host_init_port().

Implement this link training procedure for the endpoint mode as well.
Given that the RK3399 SoC does not have an interrupt signaling link
status changes, training is implemented as a delayed work which is
rescheduled until the link training completes or the endpoint controller
is stopped. The link training work is first scheduled in
rockchip_pcie_ep_start() when the endpoint function is started. Link
training completion is signaled to the function using pci_epc_linkup().
Accordingly, the linkup_notifier field of the Rockchip pci_epc_features
structure is changed to true.

Link: https://lore.kernel.org/r/20241017015849.190271-13-dlemoal@kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
[kwilczynski: update log messages to make them consistent]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

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