bus: mvebu-mbus: Add new API for window creation

We add an API to create MBus address decoding windows from the target
ID and attribute. This function will be used later and deprecate the
current name based scheme.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@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>