ARM: imx_v6_v7_defconfig: build imx sdma driver as module

Build CONFIG_IMX_SDMA as a module to avoid the following boot issue:

[    5.214751] imx-sdma 20ec000.sdma: Direct firmware load for
 imx/sdma/sdma-imx6q.bin failed with error -2
[    5.215762] imx-sdma 20ec000.sdma: Falling back to sysfs fallback
 for: imx/sdma/sdma-imx6q.bin

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>