iio: adc: Add viperboard adc driver

This adds the mfd cell to use the adc part of the Nano River Technologies
viperboard.

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>