dmaengine: dw: provide probe(), remove() stubs for users

Some users consider DMA optional, thus when driver is not compiled we shouldn't
prevent compilation of the users. Add stubs for dw_dma_probe() and
dw_dma_remove().

Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>