dma: Kconfig: Include mx6 in the IMX_SDMA help section

MX6 processors also use the IMX_SDMA driver, so include it in the help text.

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