config/pipeline: enable mediatek tree and build configs

Monitor linux `mediatek` tree. Add build configs for `for-next`
branch.

Signed-off-by: Jeny Sadadia <[email protected]>

Author: Jeny Sadadia

config/pipeline.yaml

@@ -278,6 +278,9 @@ trees:
   next:
     url: 'https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git'
 
+  mediatek:
+    url: 'https://git.kernel.org/pub/scm/linux/kernel/git/mediatek/linux.git'
+
 platforms:
 
   docker:
@@ -647,3 +650,8 @@ build_configs:
     tree: next
     branch: 'master'
     variants: *build-variants
+
+  mediatek_for_next:
+    tree: mediatek
+    branch: 'for-next'
+    variants: *build-variants