ARM: dts: imx6qdl: disable spdif "rxtx5" clock option

The spdif "rxtx5" clock option is being set to ipg clk (62) by mistake.
This causes an incorrect time keeping when spdif driver is running,
because ipg is ancestor clock for clocksource while spdif driver will
change the rate of this clock in certain circumstance.  Before the
correct clock for "rxtx5" option can be supplied, let's disable this
option for now by filling a dummy clock for it.

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>