MAINTAINERS: Update for Keystone2 secure devices

Update Keystone2 secure device configs under
"TI SYSTEM SECURITY". Without this buildman keeps complaining
about the status of these boards.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Andrew F. Davis <afd@ti.com>