ARM: dts: imx6qdl-apalis: Fix sgtl5000 detection issue

On a custom carrier board with a i.MX6Q Apalis SoM, the sgtl5000 codec
on the SoM is often not detected and the following error message is
seen when the sgtl5000 driver tries to read the ID register:

sgtl5000 1-000a: Error reading chip id -6

The reason for the error is that the MCLK clock is not provided
early enough.

Fix the problem by describing the MCLK pinctrl inside the codec
node instead of placing it inside the audmux pinctrl group.

With this change applied the sgtl5000 is always detected on every boot.

Fixes: 693e3ffa ("ARM: dts: imx6: Add support for Toradex Apalis iMX6Q/D SoM")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Acked-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>