bus: mvebu-mbus: Factor out initialization details

We introduce a common initialization function mvebu_mbus_common_init()
that will be used by both legacy and device-tree initialization code.
This patch is an intermediate step, which will allow to introduce the
DT binding for this driver in a less intrusive way.

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>