ARM: dts: imx6-tx6: disable the spi node by default

The spidev driver doesn't like to be instantiated via a naked 'spidev'
compatible, though it is very convenient to invoke it this way without
a dedicated SPI device for basic functional testing.
Disable the spi node by default to silence the WARN_ON() from the
spidev driver, but leave the configuration intact otherwise.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>