iio: dac: ltc2664: Add driver for LTC2664 and LTC2672

LTC2664 4 channel, 12-/16-Bit Voltage Output SoftSpan DAC
LTC2672 5 channel, 12-/16-Bit Current Output Softspan DAC

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Co-developed-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20240718051834.32270-7-kimseer.paller@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>