PCI: endpoint: Update documentation

PCI: endpoint: Update documentation

Document the new functions pci_epc_mem_map() and pci_epc_mem_unmap().
Also add the documentation for the functions pci_epc_map_addr() and
pci_epc_unmap_addr() that were missing.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20241012113246.95634-5-dlemoal@kernel.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>