iommu/io-pgtable: Improve documentation of .map_sg callback

The return value of the .map_sg callback is currently undocumented.
Document it.

Change-Id: I195cbfb78eaf173d916dd03190f89b2fe04e144e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>