dt-bindings: gpio: Binding for Realtek Otto GPIO

Add a binding description for Realtek's GPIO controller found on several
of their MIPS-based SoCs (codenamed Otto), such as the RTL838x and
RTL839x series of switch SoCs.

A fallback binding 'realtek,otto-gpio' is provided for cases where the
actual port ordering is not known yet, and enabling the interrupt
controller may result in uncaught interrupts.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>