PCI: rockchip: Add missing fields descriptions for struct rockchip_pcie_ep

PCI: rockchip: Add missing fields descriptions for struct rockchip_pcie_ep

When compiling the Rockchip endpoint driver with -W=1, the following
warnings can be seen in the output:

  drivers/pci/controller/pcie-rockchip-ep.c:59: warning: Function parameter or struct member 'perst_irq' not described in 'rockchip_pcie_ep'
  drivers/pci/controller/pcie-rockchip-ep.c:59: warning: Function parameter or struct member 'perst_asserted' not described in 'rockchip_pcie_ep'
  drivers/pci/controller/pcie-rockchip-ep.c:59: warning: Function parameter or struct member 'link_up' not described in 'rockchip_pcie_ep'
  drivers/pci/controller/pcie-rockchip-ep.c:59: warning: Function parameter or struct member 'link_training' not described in 'rockchip_pcie_ep'

Fix these warnings by adding the missing field descriptions in
struct rockchip_pcie_ep kernel-doc comment.

Fixes: a7137cbf6bd5 ("PCI: rockchip-ep: Handle PERST# signal in EP mode")
Fixes: bd6e61df4b2e ("PCI: rockchip-ep: Improve link training")
Link: https://lore.kernel.org/r/20241216133404.540736-1-dlemoal@kernel.org
Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>