MAINTAINERS: add Barry as a THP reviewer

I have been actively contributing to mTHP and reviewing related patches
for an extended period, and I would like to continue supporting patch
reviews.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Barry Song <[email protected]>
Acked-by: Zi Yan <[email protected]>
Acked-by: Baolin Wang <[email protected]>
Acked-by: Dev Jain <[email protected]>
Acked-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Ryan Roberts <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Nico Pache <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Barry Song

MAINTAINERS

@@ -15919,6 +15919,7 @@ R:	Liam R. Howlett <[email protected]>
 R:	Nico Pache <[email protected]>
 R:	Ryan Roberts <[email protected]>
 R:	Dev Jain <[email protected]>
+R:	Barry Song <[email protected]>
 L:	[email protected]
 S:	Maintained
 W:	http://www.linux-mm.org