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>

arch/arm/configs/imx_v6_v7_defconfig

@@ -324,6 +324,7 @@ CONFIG_USB=y
 CONFIG_USB_ANNOUNCE_NEW_DEVICES=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_EHCI_MXC=y
+CONFIG_USB_ACM=m
 CONFIG_USB_STORAGE=y
 CONFIG_USB_CHIPIDEA=y
 CONFIG_USB_CHIPIDEA_UDC=y