boards: add maintainer to the platform yaml file

Each board needs to have a maintainer to allow automatic assignment and
adding reviewers when pull requests concerning the platform are being
submitted.

This is now a placeholder so we can start adding maintainers early, the
data is not being used anywhere yet.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

scripts/schemas/twister/platform-schema.yaml

@@ -13,6 +13,10 @@ type: map
 mapping:
   "identifier":
     type: str
+  "maintainers":
+    type: seq
+    seq:
+      - type: str
   "name":
     type: str
   "type":