dt-bindings: net: marvell,prestera: Describe PCI devices of the prestera family

Even though the devices have very little in common beside the name and
the main "switch" feature, Marvell Prestera switch family is also
composed of PCI-only devices which can receive additional static
properties, like nvmem cells to point at MAC addresses, for
instance. Let's describe them.

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