dt-bindings: net: marvell,prestera: Convert to yaml

The currently described switch family is named AlleyCat3, it is a memory
mapped switch found on Armada XP boards.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>