ARM: dts: imx6: remove wrong spdif rxtx2 clock

The rxtx2 clock of i.MX6 S/PDIF is currently set to "asrc" clock.
However, according to SoC documentation, rxtx2 is connected to
ASRC_EXT_CLK, a different external clock.

Testing on Cubox-i system seems to confirm that: when fsl_spdif is
forced to select rxtx2 as input clock, audio playback fails with an I/O
error.

Set rxtx2 to the dummy clock by default to prevent fsl_spdif from
selecting it.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Cc: Mark Brown <broonie@kernel.org>
Acked-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>