iommu/arm-smmu: add DT option to avoid enabling translations on attach

There are certain use cases that require the stream matching table to be
programmed without actually enabling translations on the
SMMU (i.e. leaving SCR0.M=0).  For example, when a hypervisor is
controlling the stage-2 context bank of a nested configuration where
stage-1 needs to be bypassed.  This mode of operation is described in
the ARM SMMU spec as "stage 1 and stage 2 contexts are valid, but the
SMMU is not enabled for stage 1 translation" (Section 2.1: "Overview of
SMMU operation").

The easiest way to get the stream-matching table programmed correctly is
to program it as usual from Linux but just leave SCR0.M=0.  Add a DT
option to do this.

Change-Id: I065a38f845ae8873bc51221fe64a39b1908032d6
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>