dmaengine: Add driver for IMG MDC

Add support for the IMG Multi-threaded DMA Controller (MDC) found on
certain IMG SoCs.  Currently this driver supports the variant present
on the MIPS-based Pistachio SoC.

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