Enable support for restricted DMA pools which provide a level of DMA
memory protection on systems with limited hardware protection
capabilities, such as those lacking an IOMMU.
For instance, mt8192-asurada-spherion makes use of this to provide a
restricted DMA region for WiFi since its MT7921E WiFi card is connected
through PCIe, and the MT8192 SoC doesn't have an IOMMU context for the
PCIe controller.
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@xxxxxxxxxxxxx>