iommu: arm-smmu: wait for writes to complete before power off

SMMU registers are mapped as nGnRE, so with early acknolwdgement from
intermediate buffer, there is a chance of power off to happen first before
writes complete. Subsequently when the write is committed to actual
peripheral register, we may observe failure due to dependent power off. So
wait for all writes to complete before initiating a power off.

Change-Id: If0e9f47075b2e95cc5bc997b5f17bce69576cebc
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>