iommu/arm-smmu: don't enable SMMU device until probing has completed

We currently reset and enable the SMMU before the device has finished
being probed, so if we fail later on (for example, because we couldn't
request a global irq successfully) then we will leave the device in an
active state.

This patch delays the reset and enabling of the SMMU hardware until
probing has completed.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>