dt-bindings: ata: Add 'ata-generic' binding

The 'ata-generic' binding has been around since 2008, but never
documented.

Cc: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20221011135849.2785834-1-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>