ARM: imx_v6_v7_defconfig: Select the wm8960 codec driver

Select CONFIG_SND_SOC_WM8960 so that we can have audio functional
by default on imx6ul-evk and imx7-sdb boards.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>