dt-bindings: input: Add docs for ADC driven joystick

Add documentation for the adc-joystick driver, used to provide support
for joysticks connected over ADC.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200927123302.31062-1-contact@artur-rojek.eu


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>