PCI: rockchip: Remove redundant PCIe message routing definitions

The Rockchip driver contains duplicated message routing and INTx code
definitions (e.g., ROCKCHIP_PCIE_MSG_ROUTING_TO_RC,
ROCKCHIP_PCIE_MSG_CODE_ASSERT_INTA). These are already provided by the
PCI core in drivers/pci/pci.h as PCIE_MSG_TYPE_R_RC and
PCIE_MSG_CODE_ASSERT_INTA, respectively.

Remove the driver-specific definitions in favor of the core definitions
instead. This aligns the driver with the PCIe specification and reduces
maintenance overhead.

Signed-off-by: Hans Zhang <18255117159@163.com>
[mani: commit message rewording]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250607154913.805027-3-18255117159@163.com