iio: adc: mcp3911: add support for buffers

Add support for buffers to make the driver fit for more use cases.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220815061625.35568-6-marcus.folkesson@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>