arm64: dts: marvell: armada-37xx: mark the gpio controllers as irq controller

This allows to reference these gpio controller as interrupt parent. Also
add a comment which cpu line names are managed by the controllers
because "nb" and "sb" usually doesn't appear in schematics, but MPPX_Y
do.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>