MAINTAINERS: add Matt Mackall to embedded maintainers

Impact: make more work for myself

Signed-off-by: Matt Mackall <mpm@selenic.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Acked-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>