iio: adc: intel_mrfld_adc: Add Basin Cove ADC driver

Exposes the ADC device present on, at least, Intel Merrifield platform.

Based on work done by:
  Yang Bin <bin.yang@intel.com>
  Huiquan Zhong <huiquan.zhong@intel.com>
  Sumeet Pawnikar <sumeet.r.pawnikar@intel.com>
  Pavan Kumar S <pavan.kumar.s@intel.com>

Though it has been heavily rewritten for upstream.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>