dmaengine: Add binding document for IMG MDC

Add a binding document for the IMG Multi-threaded DMA Controller (MDC)
present on the MIPS-based Pistachio and other IMG SoCs.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>