iio: DAC extension for ltc2634-12/10/8

This patch add support for Analog Devices (Linear Technology)
LTC2634 Quad 12-/10-/8-Bit Rail-to-Rail DAC.
The SPI functionality based on them from LTC2632 therefor
add the definitions only and update the Kconfig.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>

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