iio: adc: Add MEN 16z188 ADC driver

Add support for MEN 16z188 ADC IP Core on MCB FPGAs.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@men.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>