: ad7879 - add support for AD7889

The AD7889 is a new part but 100% software compatible with the AD7879, so
add it to the id_table and help text.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
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