pcie: endpoint: Add public API for data transfer with System DMA

For a given PCIe EP device, data transfer to/from Host memory can be
achieved with "System DMA" between mapped Host memory
(PCIe outbound memory) and EP device's local memory if EP is equipped
with a "System DMA controller".
Add public API to enable such DMA transfers.

The term "System DMA" is used to clarify that we are not talking about
dedicated "PCIe DMA"; rather the one which does not understand PCIe
address directly, and uses the mapped Host memory.

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>