dmaengine: freescale: add and export fsl_dma_external_start()

The freescale driver uses custom device control FSLDMA_EXTERNAL_START to
put the controller in external start mode.
Since we are planning to deprecate the device control, move this to exported
API. Subsequent patches will remove the FSLDMA_EXTERNAL_START

Signed-off-by: Vinod Koul <vinod.koul@intel.com>