ARM: kirkwood: Split DT and legacy MBus initialization

This commit replaces the legacy MBus initialization with the new
DT-based in Kirkwood. For boards that are not yet converted to DT,
we keep the legacy initialization.

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>