misc: pci_endpoint_test: Add doorbell test case

Add doorbell support with the help of three new registers:
PCIE_ENDPOINT_TEST_DB_BAR, PCIE_ENDPOINT_TEST_DB_ADDR, and
PCIE_ENDPOINT_TEST_DB_DATA.

The testcase works by triggering the doorbell in Endpoint by writing the
value from PCI_ENDPOINT_TEST_DB_DATA register to the address provided by
PCI_ENDPOINT_TEST_DB_OFFSET register of the BAR indicated by the
PCIE_ENDPOINT_TEST_DB_BAR register and waiting for the completion status
from the Endpoint.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
[mani: removed one spurious change and reworded the 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-v21-7-57683fc7fb25@nxp.com