cmake: mod_scmi

This change adds CMake build support for module/scmi

Change-Id: Ib7c04a427a840b6db24fcccb98f2d0211d3d4e88
Signed-off-by: Chris Kay <chris.kay@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Signed-off-by: Girish Pathak <girish.pathak@arm.com>