dt-bindings: marvell,prestera: Add common compatible string

Add "marvell,prestera" as a compatible string so that drivers can be
written to account for any prestera variant without needing to
specialise to the more specific values.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>