MAINTAINERS: comment out blank M: field

Since commit ddaf5c8f3030050fcd356a1e49e3ee8f8f52c6d4
(patman: RunPipe() should not pipe stdout/stderr unless asked),
Patman spits lots of "Invalid MAINTAINERS address: '-'"
error messages for patches with global changes.
It takes too long for Patman to process them.

Anyway, "M:    -" does not carry any important information.
Rather, it is just like a place holder in case of assigning
a new board maintainer.  Let's comment out.

This commit can be reproduced by the following command:

find . -name MAINTAINERS | xargs sed -i -e '/^M:[[:blank:]]*-$/s/^/#/'

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/board/freescale/p2020ds/MAINTAINERS b/board/freescale/p2020ds/MAINTAINERS
index 42cb18d..cb61fc5 100644
--- a/board/freescale/p2020ds/MAINTAINERS
+++ b/board/freescale/p2020ds/MAINTAINERS
@@ -1,5 +1,5 @@
 P2020DS BOARD
-M:	-
+#M:	-
 S:	Maintained
 F:	board/freescale/p2020ds/
 F:	include/configs/P2020DS.h