PCI: Add PCIE_MSG_CODE_ASSERT_INTx message macros

Add "Message Routing" and "INTx Mechanism Messages" macros to enable
a PCIe driver to send messages for INTx Interrupt Signaling.

Values from PCIe r6.1, sec 2.2.8 and 2.2.8.1.

Link: https://lore.kernel.org/linux-pci/20240418-pme_msg-v8-1-a54265c39742@nxp.com


Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-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>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>