iio: amplifiers: ad8366: Add support for ADL5240 VGA

iio: amplifiers: ad8366: Add support for ADL5240 VGA

The ADL5240 is a high performance, digitally controlled variable gain
amplifier (VGA) operating from 100 MHz to 4000 MHz. The VGA integrates a
high performance, 20 dB gain, internally matched amplifier (AMP) with a
6-bit digital step attenuator (DSA) that has a gain control range of
31.5 dB in 0.5 dB steps with ±0.25 dB step accuracy.

Datasheet link:
  https://www.analog.com/media/en/technical-documentation/data-sheets/adl5240.pdf

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>