ARM: imx_v6_v7_defconfig updates

The rtc isl1208 driver is used by mx6 nitrogen board, so let's enable it by
default.

The fsl sai driver is used by the vf610-twr board, so let's enable it by
default.

simple-audio-card driver is used by the vf610-twr board, so let's enable it by
default.

Generated this patch by doing:

- make imx_v6_v7_defconfig
- make menuconfig and manually select options
- make savedefconfig
- cp defconfig arch/arm/configs/imx_v6_v7_defconfig

,which results in some additional cleanups.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>