dt-bindings: serial: Move Marvell compatible string to 8250 binding doc

These ports are compatible with NS8250 and handled by the same driver.
Get rid of the extra document that fails to document the properties that
are actually supported.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Rob Herring <robh@kernel.org>