arm: mm: dma-mapping: Use iommu_attach_group

pci devices may have multiple devices in the same group.
Only one device from the group should call arm_iommu_attach_device,
in order to prevent overriding the first device's configuration.

Change-Id: I883d67a5b42991854b00bef0ea4774f24a55e25e
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>