PCI: endpoint: pci-epf-vntb: Implement .get_dma_dev()

PCI: endpoint: pci-epf-vntb: Implement .get_dma_dev()

When vNTB is used as a PCI endpoint function, the NTB device is backed
by a virtual PCI function. For DMA API allocations and mappings, NTB
clients must use the device that is associated with the IOMMU domain.

Implement ntb_dev_ops->get_dma_dev() for pci-epf-vntb and return the EPC
parent device.

Suggested-by: Frank Li <Frank.Li@nxp.com>
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>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260306031443.1911860-4-den@valinux.co.jp