iio: adc: Add MAX1241 driver

Add driver for the Maxim MAX1241 12-bit, single-channel ADC.

Datasheet: https://datasheets.maximintegrated.com/en/ds/MAX1240-MAX1241.pdf


Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Alexandru Lazar <alazar@startmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>