iommu: msm: Correct context bank count log shown in bootup logs

for_each_child_of_node function just counts number
of child nodes mentioned in device tree. It doesn't
consider if the status of child node is disabled.

Use for_each_available_child_of_node instead to
account for correct count of context banks in SMMU
because its possible that some of the context bank
might have status disabled.

Change-Id: I89585c48511ebca3f7570f28603cc866d6af8d0d
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>