Add support for the Texas Instruments ADC081C021/027, ADC101C021/027
and ADC121C021/027 single-channel I2C ADCs. The six parts share one
register map and differ only in the resolution of the left aligned
conversion result, so one driver handles all of them.
Signed-off-by: JaeHwan Jin <jaehwan.jin@rakwireless.com>
