iio: amplifiers: ad8366: add device tree support

device-tree support achieved dropping the enum ID in favor of an extended
chip info table:
- gain step and number of channels moved to chip info
- pack_code function added to describe how SPI buffer is populated
With this, switch cases on the device type were dropped:
- simplified read_raw and write_raw callbacks
- number of channels retrieved from chip info in probe function

Signed-off-by: Rodrigo Alencar <[email protected]>

Author: rodrigo455