PCI: endpoint: Add kernel-doc for pci_epc_mem_init() API

Add missing kernel-doc for pci_epc_mem_init() API.

Link: https://lore.kernel.org/linux-pci/20230717065459.14138-8-manivannan.sadhasivam@linaro.org


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>