dt-bindings: iio: ltc2632: expand for ltc2636 support

The ltc2636 family of devices is register compatible with the ltc2636
chips, it just features 8 instead of 2 channels.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>