PCI: endpoint: pci-ep-msi: Add embedded doorbell fallback

Some endpoint platforms cannot use platform MSI / GIC ITS to implement
EP-side doorbells. In those cases, EPF drivers cannot provide an
interrupt-driven doorbell and often fall back to polling.

Add an "embedded" doorbell backend that uses a controller-integrated
doorbell target (e.g. DesignWare integrated eDMA interrupt-emulation
doorbell).

The backend locates the doorbell register and a corresponding Linux IRQ
via the EPC aux-resource API. If the doorbell register is already
exposed via a fixed BAR mapping, provide BAR+offset. Otherwise provide
the DMA address returned by dma_map_resource() (which may be an IOVA
when an IOMMU is enabled) so EPF drivers can map it into BAR space.

When MSI doorbell allocation fails with -ENODEV,
pci_epf_alloc_doorbell() falls back to this embedded backend.

Suggested-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260414141514.1341429-8-den@valinux.co.jp

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