bus: mvebu-mbus: Introduce device tree binding

This patch adds the most fundamental device-tree initialization.
We only introduce what's required to be able to probe the mvebu-mbus
driver from the DT. Follow-up patches will extend the device tree binding,
allowing to describe static address decoding windows.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
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>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>