dt-bindings: net: dsa: add MT7530 GPIO controller binding

Add device tree binding to support MT7530 GPIO controller.

Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>