Documentation: document dma device use for mcb

Hannes reported a problem with setting up dma transfers on a mcb device.
The problem boiled down to the use of a wrong 'device' for the dma
functions.

Document how to setup dma transfers for a IP core on a mcb carrier.

Reported-by: Hannes Duerr <Hannes.Duerr@duagon.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/3bdc8f76b30c2b0e2a2bfab06c2e73797ddc9384.1608305690.git.johannes.thumshirn@wdc.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>