arm64: dts: meson: g12a: add the GPIO interrupt controller

GPIO interrupts are used for the external Ethernet RGMII PHY interrupt
line.
Add the GPIO interrupt controller so we can describe that connection in
the dts files.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>