staging:iio:imu:adis16400 add support for adis16300

Next patch will remove original driver.
Note this leaves holes in the scan indexing.

Untested - except via adis16350...

V3: rebase fixup.
V2: move to single IIO_CHAN macro.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>