dt-bindings/interrupt-controller: Update Marvell ICU bindings

Change the documentation to reflect the new bindings used for Marvell
ICU. This involves describing each interrupt group as a subnode of the
ICU node. Each of them having their own compatible.

The DT binding documentation still documents the legacy binding, where
there was a single node with no subnode.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>