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>

arch/arm/configs/imx_v6_v7_defconfig

@@ -2,7 +2,7 @@ CONFIG_KERNEL_LZO=y
 CONFIG_SYSVIPC=y
 CONFIG_NO_HZ=y
 CONFIG_HIGH_RES_TIMERS=y
-CONFIG_PREEMPT_VOLUNTARY=y
+CONFIG_PREEMPT=y
 CONFIG_IKCONFIG=y
 CONFIG_IKCONFIG_PROC=y
 CONFIG_LOG_BUF_SHIFT=18