serial: Move Marvell UART DT bindings to correct location

All other UART DT binding documentation is under
Documentation/devicetree/bindings/serial/.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>