ARM: configs: imx_v6_v7_defconfig: enable USB ACM

It is used for USB CDC ACM function.

Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>