PCI: endpoint: pci-epf-test: Add doorbell test support

Add doorbell support by allocating a dedicated BAR using the
pci_epf_alloc_doorbell() API and mapping the Endpoint MSI controller
message data address to it. The data to be written in the message address
is stored in the 'pci_epf_test_reg::doorbell_data' register. Finally, the
RC can trigger doorbell in the Endpoint by writing the content of
'doorbell_data' register to the offset specified in 'doorbell_offset' of
the 'doorbell_bar' BAR.

Triggering of the doorbell is detected by pci_epf_test_doorbell_handler(),
which is bound to the doorbell IRQ. On successful completion,
STATUS_DOORBELL_SUCCESS status is set in the above mentioned handler.

To avoid breaking compatibility between host and endpoint, add two new
commands: COMMAND_ENABLE_DOORBELL and COMMAND_DISABLE_DOORBELL.

The doorbell is allocated when COMMAND_ENABLE_DOORBELL command is called
and destroyed when COMMAND_DISABLE_DOORBELL is called.

This doorbell feature only works when both RC and EP drivers support it.
If one of them doesn't support the feature, the testcase will fail.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[mani: code cleanups and reworded commit message]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20250710-ep-msi-v21-6-57683fc7fb25@nxp.com