staging:iio: dummy driver additions to show shared_by_dir infomask usage

Note that as this driver has output as well as output channels an
example of info_mask_shared_by_all that makes any sense does not exist.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>

diff --cc drivers/staging/iio/iio_simple_dummy.c
Simple merge