dma: mxs-dma: remove code left from generic DMA binding conversion

With all mxs-dma clients moved to use generic DMA helper, the code
left from generic DMA binding conversion can be removed now.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>