Enable SND_SOC_SPDIF in imx_v6_v7_defconfig to support SPDIF audio.

With commit d469b771 ("ARM: dts: imx6: update spdif sound card node
properties"), the more generic audio-codec property is used instead of
the old spdif-controller property. Since most i.MX6 boards now use the
audio-codec property together with the linux,spdif-dit and
linux,spdif-dir compatible driver, it makes sense to enable
SND_SOC_SPDIF in the imx_v6_v7_defconfig. This will ensure compatibility
with the updated device tree.

Without this change, boards that use the audio-codec property will show
the following error message during boot when using the
imx_v6_v7_defconfig and spdif audio is not working:
[   24.165534] platform sound-spdif: deferred probe pending: fsl-asoc-card: snd_soc_register_card failed

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>