Build CONFIG_IMX_SDMA as a module to allow the SDMA firmware be
retrieved after the rootfs is mounted.

This change aligns with the one already done for imx_v6_v7_defconfig
in commit 5a7374ec ("ARM: imx_v6_v7_defconfig: build imx sdma driver
as module").
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>