Let audio support be built by default.

As sgtl5000 codec is connected via I2C, let MXS I2C driver also be built
by default.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>