bus: mvebu-mbus: Add devicetree binding

Introduce the devicetree binding for the mvebu MBus driver
avaiable in the mvebu SoCs (Armada 370/XP, Kirkwood, Dove, ...).

This binding provides an accurate model of the SoC address space,
and allows to declare the address and size of the decoding windows the MBus
needs to access the peripherals, together with the target ID and attribute
for those windows.

The binding is composed of two required nodes: one for the MBus bus
and one for the MBus controller.

Cc: devicetree@vger.kernel.org
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>