MAINTAINERS: comment out blank M: field

Since commit ddaf5c8f
(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>