dma: imx-sdma: use module_platform_driver for SDMA driver

Currently there is no module_exit declared in SDMA driver, so that once
sdma module is inserted, it's shown with permanent attribute by lsmod,
and it can't be removed.
Use module_platform_driver to register/unregister SDMA driver and modify
SDMA's remove operation, to make SDMA driver possible to be removed.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>