iio: dac: ltc2632: add support for LTC2654 DAC family

iio: dac: ltc2632: add support for LTC2654 DAC family

Add support for the Linear Technology LTC2654 quad DAC family.

The LTC2654 is a 4-channel, 16-/12-bit DAC with SPI interface,
sharing the same 24-bit SPI protocol as the existing LTC2632/
LTC2634/LTC2636 devices supported by this driver.

The 12-bit variants of LTC2654 reuse existing LTC2634 chip_info
structs as they are register-compatible.

Add support for the following variants:
- LTC2654L-16: 16-bit, 2.5V internal reference
- LTC2654L-12: 12-bit, 2.5V internal reference
- LTC2654H-16: 16-bit, 4.096V internal reference
- LTC2654H-12: 12-bit, 4.096V internal reference

Signed-off-by: David Marinovic <david.marinovic@pupin.rs>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>