arm: mach-mvebu: add documentation for new device tree bindings

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Lior Amsalem <alior@marvell.com>
Tested-by: Yehuda Yitschak <yehuday@marvell.com>
Tested-by: Lior Amsalem <alior@marvell.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>