dt-bindings: interrupt-controller: Add DT binding for meson GPIO interrupt controller

This commit adds the device tree bindings description for Amlogic's GPIO
interrupt controller available on the meson8b, gxbb and gxl SoC families

Cc: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>