ARM: dts: imx6qdl: Fix SPDIF regression

Commit 833f2cbf ("ARM: dts: imx6: change the core clock of spdif")
changed many more clocks than only the SPDIF core clock as stated in
the commit message.

The MLB clock has been added and this causes SPDIF regression as
reported by Xavi Drudis Ferran and also in this forum post:
https://forum.digikey.com/thread/34240

The MX6Q Reference Manual does not mention that MLB is a clock related
to SPDIF, so change it back to a dummy clock to restore SPDIF
functionality.

Thanks to Ambika for providing the fix at:
https://community.nxp.com/thread/387131



Fixes: 833f2cbf ("ARM: dts: imx6: change the core clock of spdif")
Cc: <stable@vger.kernel.org> # 4.4.x
Reported-by: Xavi Drudis Ferran <xdrudis@tinet.cat>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Xavi Drudis Ferran <xdrudis@tinet.cat>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>