ARM: dts: imx6ul-tx6ul: Replace deprecated spi-gpio properties

As stated in Documentation/devicetree/bindings/spi/spi-gpio.yaml,
'gpio-miso', 'gpio-mosi' and 'gpio-sck' are deprecated properties.

Use the recommeded 'miso-gpios', 'mosi-gpios' and 'sck-gpios' instead.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>