PCI: cadence: Remove duplicate message code definitions

The Cadence PCIe controller driver defines message codes in enum
cdns_pcie_msg_code duplicating the existing PCIE_MSG_CODE_* definitions in
drivers/pci/pci.h. The driver only uses ASSERT_INTA and DEASSERT_INTA codes
from this enum.

Remove the redundant Cadence-specific enum definitions and use the ones
available in drivers/pci/pci.h. This helps in avoiding code duplication,
maintaining consistency with the spec, and simplifying the code
maintenance.

Signed-off-by: Hans Zhang <18255117159@163.com>
[mani: commit message rewording]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250401145023.22948-1-18255117159@163.com