drivers:dac:ltc2672: Add support for LTC2662 devices

1. Add support for LTC2662_12 and LTC2662_16 devices
2. Add support for functions ltc2672_remove_gpio(), ltc2672_cfg_reset(), ltc2672_reset(), ltc2672_write_input_register_channel(), ltc2672_write_input_register_all_channels(), ltc2672_update_channel(), ltc2672_update_all_channels(), ltc2672_hw_ldac_update()
3. Generate 32 bits command instead of 24 bits

Signed-off-by: Anush Shetty <anush.shetty@analog.com>

Author: Shetty-Anush