checkpatch: warn when a MAINTAINERS entry isn't [A-Z]:\t

For consistency, MAINTAINERS entries should be an upper case letter,
then a colon, then a tab, then the value.

Warn when an entry doesn't have this form.  --fix it too.

Link: http://lkml.kernel.org/r/9aaaf03ec10adf3888b5e98dd2176b7fe9b5fad8.1496343345.git.joe@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>