ARM: imx_v6_v7_defconfig: Select HDMI audio support

Select CONFIG_DRM_DW_HDMI_AHB_AUDIO so that we have HDMI audio
supported by default.

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