ARM: imx: cm-fx6: reinstate USB support by syncing the devicetree with Linux

ARM: imx: cm-fx6: reinstate USB support by syncing the devicetree with Linux

Commit 5248930ebf48 ("dm: imx: cm_fx6: Enable more driver model support")
enabled driver model support for USB, thereby effectively removing USB
support because the cm_fx6 devicetree in the U-Boot does *not* enable the
USB nodes.

Reinstate the USB support by syncing the devicetree with Linux whose
devicetree enables the USB nodes properly.

More precisely, use the devicetree found in Linux v4.15-rc1 with the
following two changes:
  1) Remove the audio mux; the required dt-bindings header is not
     present in the U-Boot.
  2) Keep the usdhc3 MMC controller node currently present in the
     U-Boot's devicetree to retain the ability to boot from MMC.

Fixes: 5248930ebf48 ("dm: imx: cm_fx6: Enable more driver model support")
Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>