iommu/arm-smmu: Don't fail device attach if already attached to a domain

The ARM SMMU attach_dev implementations returns -EEXIST if the device
being attached is already attached to a domain. This doesn't play nicely
with the default domain, resulting in splats such as:

  WARNING: at drivers/iommu/iommu.c:1257
  Modules linked in:

  CPU: 3 PID: 1939 Comm: virtio-net-tx Tainted: G S              4.5.0-rc4+ #1
  Hardware name: FVP Base (DT)
  task: ffffffc87a9d0000 ti: ffffffc07a278000 task.ti: ffffffc07a278000
  PC is at __iommu_detach_group+0x68/0xe8
  LR is at __iommu_detach_group+0x48/0xe8

This patch fixes the problem by forcefully detaching the device from
its old domain, if present, when attaching to a new one. The unused
->detach_dev callback is also removed the iommu_ops structures.

Signed-off-by: Will Deacon <will.deacon@arm.com>