PCI: xilinx-nwl: Enable coherent PCIe DMA traffic using CCI

PCI: xilinx-nwl: Enable coherent PCIe DMA traffic using CCI

Add support for routing PCIe DMA traffic coherently when
Cache Coherent Interconnect (CCI) is enabled in the system.
The "dma-coherent" property is used to determine if CCI is enabled
or not.
Refer to https://developer.arm.com/documentation/ddi0470/k/preface
for the CCI specification.

Link: https://lore.kernel.org/r/20210222084732.21521-1-bharat.kumar.gogada@xilinx.com
Signed-off-by: Bharat Kumar Gogada <bharat.kumar.gogada@xilinx.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>