ARM: dts: imx6: remove non-working spdif rxtx4 and rxtx6 clocks

The S/PDIF rxtx4 and rxtx6 clock inputs are "ESAI_HCKT" and "MLB clock",
respectively, according to the SoC documentation, and they are currently
mapped to clocks "esai" and "mlb".

However, they do not seem to actually work correctly. Testing on a
Cubox-i system with fsl_spdif driver forced to select one of those as
input will result in I/O errors on audio playback, which I believe means
missing clock signal.

Possibly the "ESAI_HCKT" and "MLB clock" refer to some other clocks
related to ESAI and MLB, or we are missing something else.

Since audio playback will not work if fsl_spdif selects these clocks
(which happens rarely), set the inputs do dummy clocks, at least for
now.

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>