ARM: dts: imx6qdl-nitrogen6_max: fix sgtl5000 pinctrl init

This patch fixes the following error:
sgtl5000 0-000a: Error reading chip id -6
imx-sgtl5000 sound: ASoC: CODEC DAI sgtl5000 not registered
imx-sgtl5000 sound: snd_soc_register_card failed (-517)

The problem was that the pinctrl group was linked to the sound driver
instead of the codec node. Since the codec is probed first, the sys_mclk
was missing and it would therefore fail to initialize.

Fixes: b32e7002

 ("ARM: dts: imx: add Boundary Devices Nitrogen6_Max board")
Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>