iio: adc: ad4630: Add support for ADAQ4224

ADAQ42xx devices have a PGIA gain circuitry that allows the user to
configure a set of predefined gain values through the A0 and A1 pins.

Extend driver to support ADAQ4224, ADAQ4220 and ADAQ4216 devices by
adding proper compatible match strings and control logic for setting the
PGIA gain.

Co-developed-by: Liviu Adace <[email protected]>
Signed-off-by: Liviu Adace <[email protected]>
Signed-off-by: Marcelo Schmitt <[email protected]>