iio:dac:ad7293: add support for AD7293

iio:dac:ad7293: add support for AD7293

The AD7293 is a Power Amplifier drain current controller
containing functionality for general-purpose monitoring
and control of current, voltage, and temperature, integrated
into a single chip solution with an SPI-compatible interface.

Datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/AD7293.pdf

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Cai Huoqing <cai.huoqing@linux.dev>
Link: https://lore.kernel.org/r/20211202150819.24832-1-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>