MAINTAINERS: update Documentation file entry of GPIO subsystem

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>