bus: mvebu-mbus: Restore checking for coherency fabric hardware

The new mvebu-mbus driver was not checking the device tree for
coherency fabric hardware and hence was not setting the hw_io_coherency
flag in mbus_state. This prevented the mvsdio driver from operating
correctly. This patch restores the check.

Signed-off-by: Neil Greatorex <neil@fatboyfat.co.uk>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>