dma: imx-sdma: Staticize sdma_driver_data structures

Sparse report the following warnings:

drivers/dma/imx-sdma.c:330:25: warning: symbol 'sdma_imx31' was not declared. Should it be static?
drivers/dma/imx-sdma.c:351:25: warning: symbol 'sdma_imx25' was not declared. Should it be static?
drivers/dma/imx-sdma.c:357:25: warning: symbol 'sdma_imx35' was not declared. Should it be static?
drivers/dma/imx-sdma.c:375:25: warning: symbol 'sdma_imx51' was not declared. Should it be static?
drivers/dma/imx-sdma.c:395:25: warning: symbol 'sdma_imx53' was not declared. Should it be static?
drivers/dma/imx-sdma.c:414:25: warning: symbol 'sdma_imx6q' was not declared. Should it be static?

Make the sdma_driver_data structures static.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>