MAINTAINERS: update microcode patterns

Commit bad5fa63

 ("x86, microcode: Move to a proper location") moved
the files, update the pattern.

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>