nitrogen6x: add missing pinctrl to fix mmc

Since commit f7ac30b0, the pin muxing for mmc was removed from the
board file to be managed by DM_MMC which requires PINCTRL to work. It
made the change for sabrelite but nitrogen configs were forgotten.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>