Documentation: iommu: msm: Clean up description

Clean up the binding description for IOMMUv0 bindings to make
it more clear which properties are required and which properties
are optional. This will make it easier for maintainers to maintain
and modify the code.

Change-Id: Ibbd1353133815b0ffc25ae5ca4f1e789724cc4ab
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>