arm64: defconfig: build imx-sdma as a module

This avoids firmware load error and sysfs fallback reported as follows:

[    0.199448] imx-sdma 302c0000.dma-controller: Direct firmware load
 for imx/sdma/sdma-imx7d.bin failed with error -2
[    0.199487] imx-sdma 302c0000.dma-controller: Falling back to sysfs
 fallback for: imx/sdma/sdma-imx7d.bin

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