dmaengine: dw: export probe()/remove() and Co to users

The driver library functions can be used directly by the compound devices such
as ADSP or serial driver where DesignWare DMA IP is privately attached to the
main hardware.

Instead of creating a new platform device leaf they may call dw_dma_probe()
with given struct dw_dma_chip directly and make sure that the main device is
DMA capable.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>