iio: dac: ad3530r: Add driver for AD3530R and AD3531R

iio: dac: ad3530r: Add driver for AD3530R and AD3531R

The AD3530/AD3530R (8-channel) and AD3531/AD3531R (4-channel) are
low-power, 16-bit, buffered voltage output DACs with software-
programmable gain controls, providing full-scale output spans of 2.5V or
5V for reference voltages of 2.5V. These devices operate from a single
2.7V to 5.5V supply and are guaranteed monotonic by design. The "R"
variants include a 2.5V, 5ppm/°C internal reference, which is disabled
by default.

Support for monitoring internal die temperature, output voltages, and
current of a selected channel via the MUXOUT pin using an external ADC
is currently not implemented.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20250429-togreg-v7-3-0af9c543b545@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>