dt-bindings: iio: dac: ltc2632: add LTC2654 compatible strings

dt-bindings: iio: dac: ltc2632: add LTC2654 compatible strings

Add compatible strings for the LTC2654 quad-channel DAC family.

The LTC2654 devices are 4-channel, 16-/12-bit DACs with an internal
reference and SPI interface. They use the same 24-bit SPI command
format as the LTC2632/2634/2636 family.

The 16-bit variants (LTC2654-L16 and LTC2654-H16) require new
compatible strings, as no existing compatibles support 16-bit
resolution.

The 12-bit variants (LTC2654-L12 and LTC2654-H12) are register-
compatible with LTC2634-L12 and LTC2634-H12 respectively, and can
use them as fallback compatibles.

Signed-off-by: David Marinovic <david.marinovic@pupin.rs>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>