iio: adc: ti-adc128s052: Add lower resolution devices support

The adcxx4s communicates with a host processor via an SPI/Microwire Bus
interface. The device family responds with 12bit data, of which the LSB
bits are transmitted by the lower resolution devices as 0. We don't need
to mess with ADC108S102_BITS as a result for the lower resolution
devices.

I have been able to test adc102s051, hence adding just the missing
ones in that family.

Lets reuse the driver to support the family of devices with name
ADC<bb><c>S<sss>, where
* bb is the resolution in number of bits (8, 10, 12)
* c is the number of channels (1, 2, 4, 8)
* sss is the maximum conversion speed (021 for 200 kSPS, 051 for 500 kSPS
  and 101 for 1 MSPS)

Complete datasheets are available at TI's website here:
  https://www.ti.com/lit/gpn/adc<bb><c>s<sss>.pdf

Also see: drivers/hwmon/adcxx.c

Link: https://lore.kernel.org/linux-iio/20220701042919.18180-3-nm@ti.com/


Signed-off-by: Nishanth Menon <nm@ti.com>