[PATCHv2] Add ADC driver for G20 and G45 evaluation boards

[Maxime Ripard]

Hi all,

Here is the improved version of what I've been posting some time ago.
Some things changed since last version:
  - Support for Hardware triggers for the G20 has been added
  - Support for the AT91SAM9M10G45 board has been added too, both for
    software and hardware triggers
  - The driver is now in drivers/staging/iio to include all these features.

The first three patches should not have changed much since last post, so it
should be pretty ok.

Changes since v1:
  - Added DT bindings
  - Removed the clock_name field and used adc_clk instead. Used CLKDEV_CON_ID
    for the G45.
  - Misc fixes:
    * Added clk_prepare
    * Replaced kcalloc by devm_kzalloc for the trigger allocation

Thanks,
Maxime