ARM: dts: imx6qdl-nitrogen6_som2: fix sgtl5000 pinctrl init

Since the codec is probed first, the pinctrl node should be
under the codec node.

The codec init was working for this board since U-Boot was
already setting GPIO_0 as CLKO1 but better fix it anyway.

Fixes: 3faa1bb2

 ("ARM: dts: imx: add Boundary Devices Nitrogen6_SOM2 support")
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>