MAINTAINERS: Add new Industrial I/O subsystem location

Add the new out-of-staging IIO directory to the IIO MAINTAINERS file entry.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>