ti-adc081c: Add support for adc101c and adc121c

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.pdf



Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>