docs(smmu): add smmu test engine header

This header is originated from the SMMUv3TestEngine component and added
to the tree for documentation purposes. A possible future intent is to
re-use this header in the SPM SMMU test cases implementation.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I563020aa02db888396ab8631d197c21e8d483fb8
1 file changed