DT: bindings: add missing Marvell Kirkwood SoC documentation

Marvell Kirkwood SoC binding have not yet been documented. Add the
documentation including the supported SoCs and boards.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Rob Herring <robh@kernel.org>