ARM: imx_v6_v7_defconfig: add CONFIG_FW_LOADER_USER_HELPER

To support loading sdma firmware automatically from rootfs, enable
CONFIG_FW_LOADER_USER_HELPER/CONFIG_FW_LOADER_USER_HELPER_FALLBACK.
Please get sdma firmware from below link and put it into
rootfs entry -- /lib/firmware/imx/sdma/:
https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/tree/imx/sdma



Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>