MAINTAINERS: add separate entry for DRM TTM v2

AMD is the major user of TTM, so it also makes sense that we maintain
it.

v2: mention Alex git tree as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>