dmaengine: pl330: make platform data optional

The pl330 needs platform data for describing peripheral connections, but
some platforms may only support memory to memory dma channels. In this
case, we can probe for how many channels there are and don't need the
platform data.

As memcpy requests don't need channel private data to hold peripheral
info, allow private data to be NULL in this case.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Vinod Koul <vkoul@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>