MAINTAINERS: remove arch/arm/plat-s5p/

These files were merged into plat-samsung.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Cc: Ben Dooks <ben-linux@fluff.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>