ARM: dts: imx7s-warp: Let the codec control MCLK pinctrl

sgtl5000 codec needs MCLK clock to be provided so that it can
successfully read/write via I2C, so we should better let the
codec control the pinctrl for such pin.

Thanks to Stefan Agner who provided the fix on Toradex's tree:
http://git.toradex.com/cgit/linux-toradex.git/commit/?h=toradex_imx_4.1.15_1.0.0_ga-next&id=0a55a7e5f58d46eef0d4d1d357d89e36d0c32fa4



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