staging:iio:dac:ad5446: Update Kconfig entry

There is one device supported by the driver which is not listed in the Kconfig
help test. This patch adds it. Also we are past the point were we can possible
fit all devices supported by the driver in the Kconfig entry title, so just list
the initial device that was supported by this driver and note that similar
devices are supported as well.

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>