These chips have an almost identical interface but support a different
number of value bits. Datasheet links for comparison:

 * http://www.ti.com/lit/ds/symlink/adc081c021.pdf
 * http://www.ti.com/lit/ds/symlink/adc101c021.pdf
 * http://www.ti.com/lit/ds/symlink/adc121c021.pdfSigned-off-by: NCrestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>