iommu: io-pgtable-arm: add self-test for .mag_sg

io-pgtable-arm has just gotten support for .map_sg.  Add a test to the
suite of self-tests for this.

Change-Id: Iba56bb801c1f9ef151827598022411c95d389faa
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>