ARM: dts: imx6sl-evk: Add headphone detection for sound card

Headphone detection is triggered by GPIO, then driver can
switch between speaker and headphone.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>