MAINTAINERS: update picoxcell patterns

Fix the picoxcell patterns, add the dts directory too.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>