dt-bindings: gpio: Add bindings for Cadence GPIO

This patch adds a DT binding documentation for
Cadence GPIO controller.

Signed-off-by: Jan Kotas <jank@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[Removed interrupt-parent]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>