dma: imx-dma: Add oftree support

Adding devicetree support for imx-dma driver. Use driver name for
function 'imx_dma_is_general_purpose' because the devicename for
devicetree initialized devices is different.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>