PCI: dwc: Add debugfs property to provide LTSSM status of the PCIe link

Add the debugfs property to provide a view of the current link's LTSSM
status from the Root Port device.

Signed-off-by: Hans Zhang <18255117159@163.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Tested-by: Niklas Cassel <cassel@kernel.org>
Tested-by: Hrishikesh Deleep <hrishikesh.d@samsung.com>
Link: https://lore.kernel.org/r/20250223141848.231232-1-18255117159@163.com


[kwilczynski: commit log, refactor dw_ltssm_sts_string() to avoid
compilation errors on platforms that do not set CONFIG_PCIE_DW_HOST]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>