drm/amdkfd: Document and define SVM events message macro

Document how to use SMI system management interface to enable and
receive SVM events. Document SVM event triggers.

Define SVM events message string format macro that could be used by user
mode for sscanf to parse the event. Add it to uAPI header file to make
it obvious that is changing uAPI in future.

No functional changes.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>