MAINTAINERS: update MCA section

The MCA section included a file referring to Machine Check rather than
Microchannel.

Rename the section to spell out Microchannel and drop the bad file
reference.

Reported-by: Greg Pearson <greg.pearson@hp.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>

diff --cc MAINTAINERS
Simple merge