ARM: imx_v6_v7_defconfig: Enable USB_ANNOUNCE_NEW_DEVICES

This is very helpful debugging USB issues.

Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>