dma: mmp_pdma: make the controller a DMA provider

This patch makes the mmp_pdma controller able to provide DMA resources
in DT environments by providing an dma xlate function.

of_dma_simple_xlate() isn't used here, because if fails to handle
multiple different DMA engines or several instances of the same
controller. Instead, a private implementation is provided that makes use
of the newly introduced dma_get_slave_channel() call.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>