arm64: dts: imx8dxl-evk: Remove invalid SPI property

'pinctrl-assert-gpios' is not a valid property.

Remove it to fix the following schema warning:

imx8dxl-evk.dtb: spi@5a030000: Unevaluated properties are not allowed ('pinctrl-assert-gpios' was unexpected)

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