[PATCHv3] Add ADC driver for Atmel G20, G45 and X5 boards

[Maxime Ripard]

Hi all,

Here is another version of the patches to bring ADC driver to Atmel boards.
Since last version, here is what changed:
  * Rebased on top of staging-next to move out of staging. Had to change to IIO
    kfifo buffer along the way.
  * Reworked the DT bindings according to Jean-Christophe reviews
  * Made various fixes according to reviews
  * Support for X5 boards

Thanks,
Maxime