PCI: dwc: Rename and move ltssm_status_string() to pcie-designware.c

PCI: dwc: Rename and move ltssm_status_string() to pcie-designware.c

Rename ltssm_status_string() to dw_pcie_ltssm_status_string() and move it
to the common file pcie-designware.c so that this function could be used
outside of pcie-designware-debugfs.c file.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Tested-by: Richard Zhu <hongxing.zhu@nxp.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20260120-pci-dwc-suspend-rework-v4-3-2f32d5082549@oss.qualcomm.com