ARM: dts: arm: orion5x: Add DT include for mv88f5181

Common definitions for the SoC for board definitions to use.

[gregory.clement@free-electrons.com: fix commit title]
Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>