Add PCIE_MSG_CODE_PME_TURN_OFF macros to enable a PCIe host driver to send
PME_Turn_Off messages.

Link: https://lore.kernel.org/linux-pci/20240418-pme_msg-v8-4-a54265c39742@nxp.comSigned-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>