dt-bindings: input: add bindings document for ar1021_i2c driver

Add a simple binding document describing the supported devices and the
I2C bus address.

Signed-off-by: Martin Kepplinger <martin.kepplinger@ginzinger.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>