PCI: endpoint: Add RC-to-EP doorbell support using platform MSI controller

Implement the doorbell feature by mapping the EP's MSI interrupt controller
message address to a dedicated BAR.

The EPF driver should pass the actual message data to be written to the
message address by the host through implementation-specific logic.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[mani: minor code cleanups and reworded commit message]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
[bhelgaas: fix kernel-doc]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20250710-ep-msi-v21-3-57683fc7fb25@nxp.com