iio: dac: ltc2632: add support for LTC2636 family

The only difference between LTC2632 and LTC2636 is that the former has
two DAC channels while the latter has eight.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>