iio: amplifiers: ada4250: add support for ADA4250

The ADA4250 is an instrumentation amplifier with SPI/pin-strap
progammable gains that is optimized for ultra-low power systems.
With a minimum supply voltage of 1.7V, 26uA of quiescent current,
a shutdown mode, a sleep mode, and a fast wake up settling time,
ADA4250 can be power cycled on a battery powered system for even
futher savings.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20220223120112.8067-2-antoniu.miclaus@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>