iio: potentiometer: ds1803: Add support for Maxim DS3502

The DS3502 is a 7-bit, nonvolatile digital potentiometer featuring
an output voltage range of up to 15.5V. DS3502 support is added
into existing DS1803 driver.

Datasheet: https://datasheets.maximintegrated.com/en/ds/DS3502.pdf
Signed-off-by: Jagath Jog J <jagathjog1996@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220223163525.13399-7-jagathjog1996@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>