iio: dac: Add support for ltc2632 DACs

Add support for Linear Technology LTC2632 (SPI) family of·
dual 12- 10-, and 8-bits output DACs.

Signed-off-by: Maxime Roussin-Belanger <maxime.roussinbelanger@gmail.com>
Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>