Revert "arm64: dma-mapping: Attach IOMMUs as groups"

This reverts commit commit 6a709119 ("arm64: dma-mapping: Attach
IOMMUs as groups").  PCIe endpoint devices are no longer placed into
groups with the root complex device so we no longer need to attach as
groups in the DMA layer.

CRs-Fixed: 1036401
Change-Id: I953808314ed92e9da1ac15dc5be3d1d223fee188
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>