PCI: dw-rockchip: Add rockchip_pcie_get_ltssm() helper

Add a rockchip_pcie_ltssm() helper function that reads the LTSSM status.

This helper will be used in additional places in follow-up commits.

Link: https://lore.kernel.org/linux-pci/20240607-rockchip-pcie-ep-v1-v5-8-0a042d6b0049@kernel.org


Signed-off-by: Niklas Cassel <cassel@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>