LF-141: ARM: imx_v6_v7_defconfig: enable CONFIG_PREEMPT

Enable CONFIG_PREEMPT instead of CONFIG_PREEMPT_VOLUNTARY, the former
is much suitable for embed system as i.mx. Besides, that aligns with
internal legacy imx_v7_defconfig.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reviewed-by: Anson Huang <anson.huang@nxp.com>