bus: mvebu-mbus: Update main description

After replacing the MBus name-based by the new ID-based API
let's fix the general description of the driver at the beginning
of the file.

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>