ARM: mvebu: Initialize MBus using the DT binding

Now that the mbus device tree binding has been introduced, we can
switch over to it.

Also, and since the initialization of the mbus driver is quite
fundamental for the system to work properly, this patch adds a BUG()
in case mbus fails to initialize.

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>